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http://www.archive.org/details/textbookofcomparOOmill
A TEXT- BOOK OF
COMPARATIVE PHYSIOLOGY
FOR STUDENTS AND PRACTITIONERS OF
COMPARATIVE (VETERINARY) MEDICINE
BY
WESLEY MILLS
M. A., M. D., D. V. S.
PROFESSOR OF PHYSIOLOGY IN THE FACULTY OF HUMAN MEDICINE AND THE FACULTY
OF COMPARATIVE MEDICINE AND VETERINARY SCIENCE OF MC GILL
UNIVERSITY, MONTREAL ; AUTHOR OF A TEST-BOOK
OF ANIMAL PHYSIOLOGY, ETC.
WITH 476 ILLUSTRATIONS
NEW YORK
D. APPLETON AND COMPANY
LONDON: CAXTON HOUSE, PATERNOSTER SQUARE
1890
Copyright, 1890,
By D. APPLETON AND COMPANY
All rights reserved.
PREFACE
Some years of contact with students of comparative (com-
monly called veterinary) medicine, and a fair knowledge of
the actual needs of the practitioner of this department of the
medical art, have convinced me that the time has fully come
when the text-books of physiology provided for students of
human medicine, and which the former classes have hitherto
been compelled to use, should be replaced by works written
to meet their special wants and possibilities. In fact, so dif-
ferent from man are most of the animals which the veterina-
rian is called upon to treat, and therefore to understand, in
health as well as in disease, that only the absence of suitable
works of a special character can justify the use of those that
confessedly treat of man alone.
Unfortunately, till within the past year the English-speak-
ing student of comparative medicine has been without a
single work in his own language of the special character re-
quired. Within that period two have appeared — the excel-
lent but ponderous Physiology of the Domestic Animals, by
Prof. Smith, and my own Text-Book of Animal Physiology.
It has, therefore, occurred to me that a somewhat smaller
work than the latter, embodying the same plan, but with
greater specialization for ' .ie domestic animals, would com-
mend itself to both t> students and the practitioners of
comparative medicine. In my other work I have endeavored
to set before the student a short account of what has been
deemed of most importance in general biology ; to furnish a
full account of reproduction ; to apply these two depart-
ments throughout the whole of the rest of the work ; to bring
374^ • .
iv COMPARATIVE PHYSIOLOGY.
before the student enough of comparative physiology in its
widest sense to impress him with the importance of recog-
nizing that all medicine like all science is, when at its best,
comparative ; and to show that the doctrines of evolution must
apply to physiology and medicine as well as to morphology.
Comparative medicine is essentially broad. It will not do
to measure all the animals the veterinarian is called upon to
treat by the equine standard. This has been too much the
case in the past for the good even of the horse himself ; while
others, that fall to the practitioner's care, like the dog, have
been much neglected and misunderstood.
There is no more reason, theoretically, why the veterina-
rian should overlook man than that the practitioner of human
medicine should disregard the lessons to be learned from our
domestic animals ; hence the attempt has been made to exclude
references to the human subject from the volume. The stu-
dent of comparative medicine may learn, by careful observa-
tion on himself, to understand much that would otherwise
never become realized knowledge ; and this conviction has
been at the root of a large part of the advice given the stu-
dent as to how to study throughout the work.
All that relates to reproduction and breeding is, in these
days of vast stock interests, of so much practical importance,
that on this account alone the fullest treatment of the subject
seems justifiable. But, apart from this, it has become clear to
me that function as well as form can be much better and
more easily grasped when embryology is early considered.
This I have tested, with the happiest results, with my own
classes. Usually those taking up physiology for the first
time are, of course, not expected to master all the details of
embryology, but the main outlines prove as helpful as inter-
esting ; nevertheless, it is my experience that a considerable
number of first-year men are not content to be confined to
the merest rudiments of this or any other department of
physiology.
That a work written on so new a plan as my Text-Book
of Animal Physiology should have met with a reception al-
most universally favorable, both in Britain and America, in
PREFACE, v
so short a space of time, encourages me to hope for one
equally favorable for this book, which is offered to a pro-
fession from which I look for great things in the interests
both of man and the lower animals within the next few
years. The time has certainly come when medicine must
leave the narrow ruts within which it has been confined, and
become essentially comparative. To hasten that consumma-
tion, so devoutly to be wished, has been the object with which
both my earlier and the present work have been written. Un-
less the student is infused with the broad comparative spirit
in the earliest years of his studies, and guided accordingly,
there is no sure guarantee of final success in the widest sense.
My publishers again deserve my thanks for the efforts
they have made to present this work in their best form.
Wesley Mills.
Physiological Laboratory, McGill University,
Montreal, Canada, August, 1890.
CONTENTS.
PAGE
General Biology 1
Introduction 1
Tabular statement of the subdivisions of Biology ... 4
The Cell 5
Animal and vegetable cells 5
Structure of cells .......... 5
Cell-contents . . . . 7
The nucleus 8
Tissues 8
Summary 9
Unicellular Organisms (Vegetable) ....... 9
1. Yeast .9
Morphological ......... 9
Chemical . . . . . , . . 9
Physiological . . . . . . . . .10
Conclusions 10
2. Protococcus . .11
Morphological . . . . „ . „ . . 12
Physiological „ . . .12
Conclusions . .... 12
Unicellular Animals . . . . . . . . . .13
The proteus animalcule 13
Morphological 13
Physiological . . . . . . . „ .13
Conclusions . • . . . . . ■ „ .15
Parasitic Organisms . . . . . . . . .15
Fungi 15
Mucor mucedo . . . . . . . . 17
The Bacteria 18
Unicellular Animals with Differentiation of Structure . , . 21
The bell-animalcule 21
Structure . 21
Functions 23
Vlll
COMPARATIVE PHYSIOLOGY.
Multicellular Organisms . .
The fresh-water polyps .......
The Cell reconsidered .......
The Animal Body — an epitomized account of the functions of a
mal
Living and Lifeless Matter — General explanation and compari
their properties .......
Classification of the Animal Kingdom ....
Tabular statement .......
Man's place in the animal kingdom ....
The Law of Periodicity or Rhythm in Nature — Explanation
illustrations
The Law of Habit .
Its foundation
Instincts ....
The Origin of the Forms of Life
Arguments from :
Morphology
Embryology
3 and
Mimicry .
Rudimentary organs .
Geographical distribution
Paleontology .
Fossil and existing species
Progression
Domesticated animals
Summary
Reproduction
General
The ovum
The origin and development of the ovum
Changes in the ovum itself
The male cell
The origin of the spermatozoon
Fertilization of the ovum .
Segmentation and subsequent changes
The gastrula . . .
The hen's egg .
The origin of the fowl's egg
Embryonic membranes of birds
The fuital (embryonic) membrane
The allantoic cavity .
The placenta
The discoidal placenta
of mammals
PAGE
23
23
27
28
32
34
36
36
37
41
41
42
42
45
45
45
46
46
46
46
47
47
50
51
51
55
57
58
61
61
63
64
68
69
70
74
78
80
82
83
CONTENTS.
IX
The nietadiscoidal placenta
The zonary placenta .
The diffuse placenta .
The polycotyledonary placenta .
Tabulation of placentation
Microscopic structure of the placenta
Illustrations ....
Evolution .....
Summary .....
The Development of the Embryo Itself
Germ-layers ......
Origin of the vascular system .
The growth of the embryo
Development of the Vascular System in Vertebrates
The later stages of the foetal circulation .
Development of the Urogenital System
The Physiological Aspects of Development
Ovulation
Oestrum .
The nutrition of the ovum
The foetal circulation
Periods of Gestation .
Parturition .
Changes in the circulation after birth .
Coitus . . •
Organic Evolution reconsidered
The Chemical Constitution of the Animal Body
Proximate principles
General characters of protcids
Certain non-crystalline bodies
The fats
Pecilliar fats ....••
Carbohydrates ...-••
Nitrogenous metabolites ....
Non-nitrogenous metabolites
Physiological Research and Physiological Reason
The Blood
Comparative ....
Corpuscles ....
History of the blood-eells
Chemical composition of the blood
Composition of serum
Composition of the corpuscles .
The quantity and distribution of the blood
page
84
89
89
89
90
90
91, 92
93
93
95
98
102
105
108
109
112
118
120
121
123
125
127
128
129
129
137
142
144
144
145
145
146
146
147
147
148
154
154
155
158
160
161
162
163
COMPARATIVE PHYSIOLOGY.
PAGE
The coagulation of the blood „ .163
Clinical and pathological 16*7
Summary 169
The Contractile Tissues ......... 171
General 171
Comparative .......... 172
Ciliary movements 173
The irritability of muscle and nerve 175
The Graphic Method and the Study of Muscle Physiology . .176
Chronographs and various kinds of apparatus .... 176
The apparatus used for the stimulation of muscle . . . . 179
A single muscular contraction . . . . . .185
Tetanic contraction 187
The muscle-tone . . . . • . . . . .189
The changes in a muscle during contraction 189
The elasticity of muscle 189
The electrical phenomena of muscle 191
Chemical changes in muscle 192
Thermal changes in the contracting muscle 195
The physiology of nerve 196
Electrotonus 196
Pathological and clinical 197
Electrical organs 197
Muscular work 198
Circumstances influencing the character of musular and nervous ac-
tivity 199
The influence of blood-supply and fatigue 199
Separation of muscle from the central nervous system . . 201
The influence of temperature 201
Unstriped muscle 202
General 202
Comparative . . . 202
Special considerations ......... 203
Functional variations ........ 204
Summary of the physiology of muscle and nerve . . . 205
The Nervous System — General Considerations .... 208
Experimental . . . . . . . . . .210
Automatism . . . . . . . . . .211
Conclusions 212
Nervous inhibition . . . . . . . . .212
The Circulation oe the Blood 214
General 214
The mammalian heart ........ 215
Circulation in the mammal 219
CONTENTS.
XI
The action of the mammalian heart .
The velocity of the blood and blood-pressure
General .....
Comparative ....
The circulation under the microscope
The characters of the blood-flow
Blood-pressure. ....
The Heart
The cardiac movements .
The impulse of the heart .
Investigation of the heart-beat from within
The cardiac sounds ....
Causes of the sounds
Endo-cardiac pressures
The work of the heart
Variations in the cardiac pulsation .
Comparative ....
The pulse
Features of an arterial pulse-tracing
Venous pulse ....
Pathological ....
Comparative ....
The beat of the heart and its modifications
The nervous system in relation to the heart
Influence of the vagus nerve on the heart
Conclusions .....
The accelerator nerves of the heart .
The heart in relation to blood-pressure
The influence of the quantity of blood
The capillaries
Special considerations
Pathological
Personal observations
Comparative
Evolution .
Summary of the physiology of the circulat
Digestion of Food
Foodstuffs, milk, etc
Embryological .
Comparative
Structure, arrangement, and significance of the
The digestive juices .
Saliva and its action
Secretion of the different elands
teeth
PAGE
222
223
223
224
224
226
227
231
231
232
233
234
235
236
238
239
240
241
242
244
244
244
248
249
253
257
258
260
261
264
265
266
566
267
268
269
274
274
280
286
286
297
297
298
Xll
COMPARATIVE PHYSIOLOGY.
Comparative
Gastric juice
Bile .
General
Pigments .
Digestive action
Comparative
Pancreatic secretion
Succus entericus
Comparative
Secretion as a physiological process .
Secretion of the salivary glands
Secretion by the stomach .
The secretion of bile and pancreatic
The nature of the act of secretion .
Self-digestion of the digestive organs
Comparative ....
The alimentary canal of the vertebrate
The movements of the digestive organs
Deglutition
Comparative
The movements of the stomach
Comparative
Pathological
The intestinal movements .
Defecation
Vomiting ....
Comparative
The removal of digestive products f
Lymph and chyle
The movements of the lymph
Pathological
Faeces . ■» .
Pathological
The changes produced in the food in
General
Comparative
Pathological
Special considerations
Various
Evolution .
Summary .
The Respiratory System
General
juice
comparative
om the alimentary canal
the alimentary
canal
PAGE
298
299
301
301
302
303
304
305
307
310
311
311
315
316
318
323
324
331
332
333
336
336
337
337
337
338
339
340
341
343
344
352
352
353
354
354
357
359
359
359
363
364
366
366
CONTENTS,
xin
blood
espiration
Anatomical ....
The entrance and exit of air
The muscles of respiration
Types of respiration .
Comparative .....
Personal observation
The quantity of air respired
The respiratory rhythm .
General .....
Pathological . .
Respiratory sounds .
Comparison of the inspired and the expired air
Respiration in the blood .
Haemoglobin and its derivatives
General
Blood-spectra ....
Comparative ....
The nitrogen and the carbon dioxide of the
Foreign gases and respiration .
Respiration in the tissues ....
The nervous system in relation to respiration
The influence of the condition of the blood on i
The influence of respiration on the circulation
General ......
Comparative .....
The respiration and circulation in asphyxia
Pathological .....
Peculiar respiratory movements
Coughing, laughing, etc. .
Comparative .....
Special considerations ....
Pathological and clinical .
Personal observation
Evolution ......
Summary of the physiology of respiration
Protective and Excretorv Functions of the
General ......
Comparative
The excretory function of the skin .
Normal sweat .....
Pathological ......
Comparative — Respiration by the skin
Death from suppression of the functions of the
The excretion of perspiration ....
Skin
skin
PAGE
367
370
373
375
375
375
378
379
379
379
381
382
383
385
385
387
389
389
391
391
393
393
396
396
398
399
401
401
401
402
403
403
404
404
405
408
408
408
411
411
411
412
412
412
xiv COMPARATIVE PHYSIOLOGY.
PAGE
Experimental 413
Absorption by the skin 413
Comparative .......... 413
Summary ........... 413
Excretion by the Kidney - . . . . 415
Anatomical .......... 415
Comparative . 415
Urine considered physically and chemically . . . . .419
Specific gravity . . . . . . . . . .419
Color .419
Eeaction 419
Quantity . . . . . . . . . . . 419
Composition : Nitrogenous crystalline bodies .... 420
Non-nitrogenous organic bodies ...... 420
Inorganic salts . . . . . . . . , 420
Abnormal urine . . . . . . . . .421
Comparative . . . . . . . ... . 421
The secretion of urine 421
Theories of secretion . . . . ' . . . . 421
Nervous influence . . . . . . . . 423
Pathological 423
The expulsion of urine ......... 424
General .424
Facts of experiment and of experience ..... 424
Pathological ' . .426
Summary of urine and the functions of the kidneys .... 426
Comparative .......... 426
The Metabolism of the Body 428
General remarks ......... 428
The metabolism of the liver ........ 428
The glycogenic function 429
The uses of glycogen ........ 429
Metabolism of the spleen 429
Histological 430
Chemical 430
The construction of fat 432
General and experimental ....... 432
Histological . . 433
Changes in the cells of the mammary gland .... 434
Nature of fat formation 434
Milk and colostrum ......... 435
Pathological 435
Comparative .......... 436
The study of tlie metabolic processes by other methods . . . 436
CONTENTS.
xv
PAGE
Various tabular statements ....... 437
Starvation and its lessons ....... 537
Comparative 438
Diets 439
Feeding experiments ......... 439
General 439
Nitrogeneous equilibrium ........ 440
Comparative 441
The effects of gelatine in the diet 441
Fat and carbohydrates 442
Comparative .......... 442
The effects of salts, water, etc., on the diet .... 443
Patholgical 443
Water 443
The energy of the animal body I ..... 443
Tabular statements ......... 444
Animal heat 445
General ........... 445
Comparative .......... 445
The regulation of temperature 447
Cold-blooded and warm-blooded animals compared . . . 448
Theories of heat formation and heat regulation .... 448
Pathological . . . 449
Special considerations ......... 450
Evolution 450
Hibernation .......... 451
Daily variations in temperature in man and other mammals . 452
The influence of the nervous system on metabolism (nutrition). . 452
Experimental facts ° 452
Discussion of their significance 455
General considerations, clinical and pathological . . .456
Summary of metabolism ........ 458
The Spinal Cord — General 461
General 461
Anatomical .......... 461
The reflex functions of the spinal cord 466
General and experimental ........ 466
Evolution and heredity 468
Inhibition of reflexes 469
The spinal cord as a conductor of impulses 469
Anatomical 469
Decussation .......... 471
Pathological 471
Paths of impulses 473
XVI
COMPARATIVE PHYSIOLOGY.
ain with another
ng function ?
The automatic functions of the spinal cord
General .
Spinal phenomena
Special considerations
Comparative
Evolution
Synoptical
The Bkain ....
General and anatomical
Animals deprived of their cerebrum
Behavior of various animals and its significance
Have the semicircular canals a co-ordinat
Discussion of the phenomena .
Forced movements
Functions of the cerebral convolutions
Comparative ....
Individual differences in brains
The connection of one part of the br
The cerebral cortex .
Theories of different observers
The circulation in the brain
Sleep — hibernation — dreaming
Hypnotism, etc.
Illustrations of localization
Functions of other portions of the brain
The corpus striatum and the optic thalamus
Corpora quadrigemina
The cerebellum
Pathological
Crura cerebri and pons Varolii
Pathological
Medulla oblongata .
Special considerations
Embryological .
Evolution ....
Synoptical
General Remarks on the Senses
Anatomical
General principles
The Skin as an Organ ok Sense
General ....
Pressure sensations .
Thermal sensations .
Tactile sensibility
CONTENTS. xvii
PAGE
The muscular sense 524
General ........... 524
Pathological 524
Comparative . . . . . . . . . .524
Synoptical .......... 525
Vision 526
Anatomical 527
Einbryological . . . . . . . . . .528
Dioptrics of vision .......... 531
Accommodation of the eye 532
Alterations in the size of the pupil 533
Phenomena and their explanations ...... 534
Pathological 536
Comparative ....... . . 536
Optical imperfections of the eye 536
Anomalies of refraction 536
Visual sensations 538
General 538
Affections of the retina ........ 540
The laws of retinal stimulation ....... 541
The visual angle 542
Color- vision , 543
Psychological aspects of vision ....... 543
The visual field 543
After-images, etc. ......... 543
Co-ordination of the two eyes in vision ...... 544
The visual axes . . . . . . . . . 544
Ocular movements ......... 544
Estimation of the size and distance of objects .... 548
Solidity 548
Protective mechanisms of the eye ....... 549
Special considerations 551
Comparative . . . . . . . . . .551
Evolution 552
Pathological 554
Brief synopsis of the physiology of vision ..... 554
Hearing 557
General ........... 557
Anatomical .......... 558
The membrana tympani ........ 558
The auditory ossicles ........ 559
Muscles of the middle ear 561
The Eustachian tube 562
Pathological 563
B
XY1U
COMPARATIVE PHYSIOLOGY.
PAGE
Auditory sensations, perceptions, judgments
. 567
. 567
. 568
Synopsis of the physiology of hearing
. 672
The Senses of Smell and Taste
. 573
Smell
. 573
Anatomical .....
. 573
. 573
. 574
, 575
, 576
. 576
Experimental ......
. 577
. 578
The Cerebro-Spinal System of Nerves
. 579
. 579
. 5 SO
Additional experiments
. 580
. 580
. 580
. 580
The motor-oculi, or third nerve .
. 581
The trochlear, or fourth nerve .
. 582
The abducens, or sixth nerve
. 282
The trigeminus, or fifth nerve .
. 583
The glossopharyngeal, or ninth nerve
. 585
The pneumogastric, or tenth nerve .
. 586
The spinal accessory, or eleventh nerve
. 587
The hypoglossal, or twelfth nerve
. 588
Relations of the cerebrospinal and sympatheti
c syst
ems
. 588
Recent views on this subject
. 588
The Voice
. 592
Physical ......
. 592
Anatomical .....
. 593
Voice-formation ....
. 593
. 598
Oomparative .....
. 598
. 600
CONTENTS.
XIX
Evolution .
Summary .
Certain Tissues .
Connective tissue
Elastic tissue .
Bone
Cartilage .
Locomotion .
Anatomical
Mechanical
Standing .
Walking .
Running .
Jumping .
Hopping .
Comparative : the gait of
The foot of the horse
Different gaits : general
Walking .
The amble
The trot .
The gallop
Evolution .
juadruped
PAGE
600
601
602
003
604
604
604
610
610
610
610
611
613
614
614
614
616
621
623
624
624
624
627
COMPARATIVE PHYSIOLOGY.
GENERAL BIOLOGY.
INTRODUCTION.
Biology (fiios, life ; Aoyos, a dissertation) is the science which.
treats of the nature of living things ; and, since the properties
of plants and animals can not be explained without some knowl-
edge of their form, this science includes morphology (jioptfir),
form ; Aoyo?, a dissertation) as well as physiology (<]>v<ns, na-
ture ; \oyos).
Morphology describes the various forms of living things and
their parts ; physiology, their action or function.
General biology treats neither of animals nor plants exclu-
sively. Its province is neither zoology nor botany ; but it at-
tempts to define wbat is common to all living things. Its aim
is to determine the properties of organic beings as such, rather
than to classify or to give an exhaustive account of either ani-
mals or plants. Manifestly, before this can be done, living
things, both animal and vegetable, must be carefully compared,
otherwise it would be impossible to recognize differences and
resemblances ; in other words, to ascertain what they have in
common .
When only the highest animals and plants are contem-
plated, the differences between them seem so vast that they
appear to have, at first sigbt, nothing in common but that they
are living : between a tree and a dog an infant can discrimi-
nate ; but there are microscopic forms of life that tbus far defy
the most learned to say whether they belong to the animal or
the vegetable world. As we descend in the organic series, the
lines of distinction g-row fainter, till they seem finally to all but
disappear.
2 COMPARATIVE PHYSIOLOGY.
But let ns first inquire : What are the determining charac-
teristics of living things as such ? By what barriers are the
animate and inanimate worlds separated ? To decide this, falls
within the province of general biology.
Living things grow by interstitial additions of particles of
matter derived from without and transformed into their own
substance, while inanimate bodies increase in size by superficial
additions of matter over which they have no power of decompo-
sition and recomposition so as to make them like themselves.
Among lifeless objects, crystals approach nearest to living
forms ; but the crystal builds itself up only from material in
solution of the same chemical composition as itself.
The chemical constitution of living objects is peculiar. Car-
bon, hydrogen, oxygen, and nitrogen are combined into a very
complex whole or molecule, as protein ; and, when in combina-
tion with a large proportion of water, constitute the basis of all
life, animal and vegetable, known as protoplasm. Only living
things can manufacture this substance, or even protein.
Again, in the very nature of the case, protoplasm is continu-
ally wasting by a process of oxidation, and being built up from
simpler chemical forms. Carbon dioxide is an invariable prod-
uct of this waste and oxidation, while the rest of the carbon,
the hydrogen, oxygen, and nitrogen are given back to the in-
organic kingdom in simpler forms of combination than those
in which they exist in living beings. It will thus be evident
that, while the flame of life continues to burn, there is constant
chemical and physical change. Matter is being continuously
taken from the world of things that are without life, trans-
formed into living beings, and then after a brief existence in
that form returned to the source from which it was originally
derived. It is true, all animals require their food in organized
form — that is, they either feed on animal or plant forms ; but
the latter derive their nourishment from the soil and the atmos-
phere, so that the above statement is a scientific truth.
Another highly characteristic pi'operty of all living things
is to be sought in their periodic changes and very limited dura
tion. Every animal and plant, no matter what its rank in the
scale of existence, begins in a simple form, passes through a
series of changes of varying degrees of complexity, and finally
declines and dies ; which simply means that it rejoins the in-
animate kingdom : it passes into another world to which it
formerly belonged.
GENERAL BIOLOGY. 3
Living things alone give rise to living things ; protoplasm
alone can heget protoplasm ; cell begets cell. Omne animal
(anima, life) ex ovo applies with a wide interpretation to all
living forms.
From what has been said it will appear that life is a condi-
tion of ceaseless change. Many of the movements of the pro-
toplasm composing the cell-units of which living beings are
made are visible under the microscope; their united effects are
open to common observation — as, for example, in the move-
ments of animals giving rise to locomotion we have the joint
result of the movements of the protoplasm composing millions
of muscle-cells. But, beyond the powers of any microscope that
has been or probably ever will be invented, there are molecular
movements, ceaseless as the flow of time itself. All the pro-
cesses which make up the life-history of organisms involve this
molecular motion. The ebb and flow of the tide may symbolize
the influx and efflux of the things that belong to the inanimate
world, into and out of the things that live.
It follows from this essential instability in living forms that
life must involve a constant struggle against forces that tend
to destroy it; at best this contest is maintained successfully for
but a few years in all the highest grades of being. So long as
a certain equilibrium can be maintained, so long may life con-
tinue and no longer.
The truths stated above will be illustrated in the simpler
forms of plants and animals in the ensuing pages, and will
become clearer as each chapter of this work is perused. They
form the fundamental laws of general biology, and may be for-
mulated as follows:
1. Living matter or protoplasm is characterized by its chemi-
cal composition, being made up of carbon, hydrogen, oxygen,
and nitrogen, arranged into a very complex molecule.
2. Its universal and constant waste and its repair by inter-
stitial formation of new matter similar to the old.
3. Its power to give rise to new forms similar to the parent
ones by a process of division.
4. Its manifestation of periodic changes constituting devel-
opment, decay, and death.
Though there is little in relation to living beings which
may not be appropriately set down under zoology or botany, it
tends to breadth to have a science of general biologjr which
deals with the properties of things simply as living, irrespective
COMPARATIVE PHYSIOLOGY.
Biol-
ogy-
The
science
of liv-
ing i
things ;
i. e.,of
matter
in the
living
state.
f Mor-
phol-
ogy.
The
science
of
form,
struct-
ure,
etc.
Essen-
tially
statical.
Physi-
ology
The
science
of
action
or
func-
tion.
Essen-
tially
dynam-
ical.
Anatomy.
The science of structure;
the termbeiug usually
applied to the coarser
and more obvious
composition of plants
or animals.
Histology.
Microscopical anatomy.
The ultimate optical
analysis of structure
by the aid of the mi-
croscope ; separated
from anatomy only as
a matter of conven-
ience.
Taxonomy.
The classification of liv-
ing things, based
chiefly on phenomena
of structure.
Distribution.
Considers the position
of living things in
space and time ; their
distribution over the
present face of ■ the
earth; and their dis-
tribution and succes-
sion at former pe-
riods, as displayed in
i fossil remains.
Embryology.
The science of develop-
ment from the germ ;
includes many mixed
problems pertaining
both to morphology
and physiology. At
present largely mor-
phological.
Physiology.
The special science of
the functions of the
individual in health
and in disease ; hence
including Pathology.
Psychology.
The science of mental
phenomena.
Sociology.
The science of social
life, i. e., the life of
communities, wheth-
er of men or of lower
animals.
Botany.
The
science
of veg-
etal
living
matter
or
plants.
1
Biol-
ogy-
The
science
of liv-
things ;
i. e., of
matter
in the
living
state.
Zool-
ogy.
The
science
of
animal
living
matter
or ani
mals. J
GENERAL BIOLOGY. 5
very much as to whether they belong to the realm of animals or
plants. The relation of the sciences which may be regarded
as subdivisions of general biology is well shown in the accom-
panying table. *
THE CELL, f
All living things, great and small, are composed of cells.
Animals may be divided into those consisting of a single cell
(Protozoa), and those made up of a multitude of cells (Metazoa) ;
but in every case the animal begins as a single cell or ovum
from which all the other cells, however different finally from
one another either in form or function, are derived by processes
of growth and division ; and, as will be seen later, the whole
organism is at one period made up of cells practically alike in
structure and behavior. The history of each individual animal
or plant is the resultant of the conjoint histories of each of its
cells, as that of a nation is, when complete, the story of the total
outcome of the lives of the individuals composing it.
It becomes, therefore, highly important that a clear notion
of the characters of the cell be obtained at the outset ; and
this chapter will be devoted to presenting a general account of
the cell.
The cell, whether animal or vegetable, in its most complete
form consists of a mass of viscid, semifluid, transparent sub-
stance (protoplasm), a cell wall, and a more or less circular
body (nucleus) situated generally centrally within; in which,
again, is found a similar structure (nucleolus) .
This description applies to both the vegetable and the ani-
mal cell ; but the student will find that the greater proportion
of animal cells have no cell wall, and that very few vegetable
cells are without it. But there is this great difference between
the animal and vegetable cell: the former never has a cellulose
wall, while the latter rarely lacks such a covering. In every
case the cell wall, whether in animal or vegetable cells, is of
greater consistence than the rest of the cell. This is especially
true of the vegetable cell.
It is doubtful whether there are any cells without a nucleus,
while not a few, especially when young and most active, pos-
* Taken from the General Biology of Sedgwick and Wilson.
f The illustrations of the sections following will enable the student to
form a generalized mental picture of the cell in all its parts.
6
COMPARATIVE PHYSIOLOGY.
sess several. The circular form may be regarded as the typical
form of both cells and nuclei, and their infinite variety in size
and form may be considered as in great part the result of the
action of mechanical forces, such as mutual pressure ; this is, of
course, more especially true of shape. Reduced to its greatest
simplicity, then, the cell may be simply a' mass of protoplasm
with a nucleus.
It seems probable that the numerous researches of recent
years and others now in progress will open up a new world of
Fig. 1.— Nuclear division. A-II, karyokinesis of a tissue cell. A, nuclear reticulum
in its ordinary state. B, preparing for division ; the contour is less defined, and
the fibers thicker and less intricate. C, wreath-stage ; the chromatin is arranged
in a complicated looping round the equator of the achromatin spindle. D, nio-
naster-stage ; the chromatin now appears as centripetal equatorial V's, each of
which should be represented as double. B, a migration of the half of each chro-
matin loop towards opposite poles of the spindle. F, diaster-stage ; the chroma-
tin forme a star, round each pole of a spindle, each aster being connected by
Strands of achromatin. G, daughter-wreath stage; the newly formed nuclei are
passing through their retrogressive development, which is completed in the rest-
ing stage, H. d-f, karyokinesis of an egg-cell, showing the smaller amount of
chromatin than 'in the tissue-cell. The stages d, e,fi correspond to D, E, F, re-
spectively. The polar star at the end of the spindle is composed of protoplasm-
granules of the cell itself, and must not be mistaken for the diaster(F). The
coarse lii es represent the chromatin, the fine lines the achromatin, and the dotted
lines cell-gran tiles. (Chiefly modified from (Hemming.) X-Z, direct nuclear divis-
ion in the cells of the embryonic integument of the European scorpion. After
Blochmann (Haddorl).
cell biology which will greatly advance our knowledge, espe-
cially in the direction of increased depth and accuracy.
GENERAL BIOLOGY. 7
Though many points are still in dispute, it may be safely
said that the nucleus plays, in most cells, a role of the highest
importance; in fact, it seems as though we might regard the
nucleus as the directive brain, so to speak, of the individual
cell. It frequently happens that the behavior of the body of
the cell is foreshadowed by that of the nucleus. Thus fre-
quently, if not always, division of the body of the nucleus pre-
cedes that of the cell itself, and is of a most complicated char-
acter (karyokinesis or mitosis). The cell wall is of subordinate
importance in the processes of life, though of great value as a
mechanical support to the protoplasm of the cell and the aggre-
gations of cells known as tissues. The greater part of a tree
may be said to be made up of the thickened walls of the cells,
and these are destitute of true vitality, unless of the lowest
order ; while the really active, growing part of an old and large
tree constitutes but a small and limited zone, as may be learned
from the plates of a work on modern botany representing sec-
tions of the wood.
Animals, too, have their rigid parts, in the adult state espe-
cially, resulting from the thickening of a part of the whole of
the cell by a deposition usually of salts of lime, as in the case of
the bones of animals. But in some cases, as in cartilage, the
cell wall or capsule undergoes thickening and consolidation,
and several may fuse together, constituting a matrix, which is
also made up in part, possibly, of a secretion from the cell pro-
toplasm. In the outer parts of the body of animals we have a
great abundance of examples of thickening and hardening of
cells. Very well-known instances are the indurated patches
of skin {epithelium) on the palms of the hands and else-
where.
It will be scarcely necessary to remark that in cells thus
altered the mechanical has largely taken the place of the vital
in function. This at once harmonizes with and explains what is
a matter of common observation, that old animals are less act-
ive— have less of life within them, in a word, than the young.
Chemically, the cellulose wall of plant-cells consists of carbon,
hydrogen, and oxygen, in the same relative proportion as exists
in starch, though its properties are very different from those of
that substance.
Turning to cell contents, we find them everywhere made up
of a clear, viscid substance, containing almost always granules
of varying but very minute size, and differing in consistence
8 COMPARATIVE PHYSIOLOGY.
not only in different groups of cells, but often in the same cell,
so that we can distinguish an outer portion {ectoplasm) and an
inner more fluid and more granular region (endojolasm).
The nucleus is a body with very clearly defined outline (in
some cases limited by a membrane), through which an irregular
network of fibers extends that • stains more deeply than any
other part of the whole cell.
Owing to the fact that it is so readily changed by the action
of reagents, it is impossible to ascertain the exact chemical com-
position of living protoplasm ; in consequence, we can only
infer its chemical structure, etc., from the examination of the
dead substance.
In . general, it may be said that protoplasm belongs to the
class of bodies known as proteids — that is, it consists chemically
of carbon, hydrogen, a little sulphur, oxygen, and nitrogen, ar-
ranged into a very complex and unstable molecule. This very
instability seems to explain at once its adaptability for the man-
ifestation of its nature as living matter, and at the same time the
readiness with which it is modified by many circumstances, so
that it is possible to understand that life demands an incessant
adaptation of internal to external conditions.
It seems highly probable that protoplasm is not a single pro-
teid substance, but a mixture of such ; or let us rather say, fur-
nishes these when chemically examined and therefore dead.
Very frequently, indeed generally, protoplasm contains other
substances, as salts, fat, starch, chlorophyl, etc.
From the fact that the nucleus stains differently from the
cell contents, we may infer a difference between them, physi-
cal and especially chemical. It (nucleus) furnishes on analysis
nuclein, which contains the same elements as protoplasm (with
the exception of sulphur) together with phosphorus. Nuclei
have great resisting power to ordinary solvents and even the
digestive juices.
Inasmuch as all vital phenomena are associated with proto-
plasm, it has been termed the " physical basis of life " (Hux-
ley)\
Tissues. — A collection of cells performing a similar physio-
logical action constitutes a tissue.
Generally the cells are held together either by others with
that sole function, or by cement material secreted by them-
selves. An organ may consist of one or several tissues. Thus
the stomach consists of muscular, serous, connective, and gland-
GENERAL BIOLOGY. 9
ular tissues, besides those constituting its blood-vessels, lym-
phatics, and nerves. But all of the cells of each tissue have,
speaking generally, the same function. The student is referred
to works on general anatomy and histology for classifications
and descriptions of the tissues. See also page 603.
The statements of this chapter will find illustration in tbe
pages immediately following, after which we shall return to
the subject of the cell afresh.
Summary. — The typical cell consists of a wall, protoplasmic
contents, and a nucleus. The vegetable cell has a limiting
membrane of cellulose. Cells undergo differentiation and may
be united into groups forming tissues which serve one or more
definite purposes.
The chemical constitution of protoplasm is highly complex
and unstable. The nucleus plays a prominent part in the life-
history of the cell, and seems to be essential to its perfect devel-
opment and greatest physiological efficiency.
UNICELLULAR PLANTS.
Yeast (Torula, Saccharomyces Cerevisice).
The essential part of the common substance, yeast, may be
studied to advantage, as it affords a simple type of a vast group
of organisms of profound interest to the student of physiology
and medicine. To state, first, the main facts as ascertained by
observation and experiment :
Morphological. — The particles of which yeast is composed
are cells of a circular or oval form, of an average diameter of
about s-gVo of an inch.
Each individual torula cell consists of a transparent homo-
geneous covering (cellulose) and granular semifluid contents
(protoplasm). Within the latter there may be a space (vacu-
ole) filled with more fluid contents.
The various cells produced by budding may remain united
like strings of beads. Collections of masses composed of four
or more subdivisions (ascospores), which finally separate by rup-
ture of the original cell wall, having thus become themselves in-
dependent cells, maybe seen more rarely (endogenous division).
The yeast-cell is now believed to possess a nucleus.
Chemical. — When yeast is burned and the ashes analyzed,
they are found to consist chiefly of salts of potassium, calcium,
and magnesium.
10
COMPARATIVE PHYSIOLOGY.
The elements of which yeast is composed are C, H, O, N, S,
P, K, Mg, and Ca; but chiefly the first four.
Physiological. — If a little of the powder obtained by drying
yeast at a temperature below blood-heat be added to a solution
of sugar, and the lat-
ter be kept warm,
bubbles of carbon di-
oxide will be evolved,
causing the mixture
to become frothy ; and
the fluid will acquire
an alcoholic charac-
ter {fermentation).
If the mixture be
raised to the boiling-
point, the process de-
scribed at once ceases.
It may be further
noticed that in the
fermenting saccha-
rine solution there is
a gradual increase of
turbidity. All of these
changes go on per-
fectly well in the to-
tal absence of sun-
light.
Yeast - cells are
found to grow and
reproduce abundant-
ly iu an artificial food
solution consisting of
a dilute solution of
Fig. 4.— Further development of the forms represented certain Salts, together
in Fig. 3. .,, ' 6
with sugar.
Conclusions. — What are the conclusions which may be legiti-
mately drawn from the above facts ?
That the essential part of yeast consists of cells of about the
size of mammalian blood-corpuscles, but with a limiting wall
of a substance different from the inclosed contents, which latter
is composed chiefly of that substance common to all living
things — protoplasm; that like other cells they reproduce their
Fig. 2. — Various stages in the development of brewer's
yeast, seen, with the exception of the first in the
series, with an ordinary high power (Zeiss, D. 4) of
the microscope. The first is greatly magnified
(Gundlach's ^ immersion lens). The second series
of four represents stages in the division of a single
cell ; and the third series a branching colony.
Everywhere the light areas indicate vacuoles.
Fig. 3.— The cndogonidia (ascospore) phase of repro-
duction—i. e., endogenous division.
GENERAL BIOLOGY.
11
kind, and in this instance by two methods : gemmation giving
rise to the bead- like aggregations alluded to above; and in-
ternal division of the protoplasm (endogenous division).
From the circumstances under which growth and reproduc-
tion take place, it will be seen that the original protoplasm of
the cells may increase its bulk or grow when supplied with
suitable food, which is not, as will be learned later, the same in
all respects as that on which green plants thrive ; and that this
may occur in darkness. But it is to be especially noted that the
protoplasm resulting from the action of the living cells is
wholly different from any of the substances used as food. This
power to construct protoplasm from inanimate and unorgan-
ized materials, reproduction, and fermentation are all proper-
ties characteristic of living organisms alone.
It will be further observed that these changes all take place
within narrow limits of temperature; or, to put the matter
more generally, that the life-history of this humble organism
can only be unfolded under certain well-defined conditions.
Protococcus (Protococcus pluvialis).
The study of this one- celled plant will afford instructive
comparison between the ordinary green plant and the colorless
plants or fungi.
Fig. 5.
Fig. 6.
Fig. 7.
Figs. 5 to 7 represent successive stages observed in the life-history of Protococci
scraped from the bark of a tree.
Fig. 5.— A group in the dried state, illustrating method of division.
Fig. 6.— One of the above after two days' immersion in water.
Fig. 7. — Various phases in the later motile stage assumed by the above specimens.
The nucleus is denoted by nc: the cell wall by c.w ; and the coloring-matter by
the dark spot. On the left of Fig. 7 an individual may be seen that is "devoid of a
cell wall.
12 COMPARATIVE PHYSIOLOGY.
Like Tovula it is selected because of its simple nature, its
abundance, and tbe ease with which it may be obtained, for it
abounds in water-barrels, standing- pools, dri n king-troughs,
etc.
Morphological. — Protococcus consists of a structureless wall
and viscid granular contents, i. e., of cellulose and protoplasm.
The protoplasm may contain starch and a red or green color-
ing matter (chlorophyl) . It probably contains a nucleus. The
cell is mostly globular in form.
Physiological. — It reproduces by division of the original cell
(fission) into similar individuals, and by a process of budding
and constriction (gemmation) which is much rarer. Under the
influence of sunlight it decomposes carbon dioxide (CO2), fixing
the carbon and setting the oxygen free. It can flourish per-
fectly in rain-water, which contains only carbon dioxide, salts
of ammonium, and minute quantities of other soluble salts that
may as dust have been blown into it.
There is a motile form of this unicellular plant, and in this
stage it moves through the fluid in which it lives by means of
extensions of its protoplasm (cilia) through the cell wall ; or
the cell wall may disappear entirely. Finally, the motile form,
withdrawing its cilia and clothing itself with a cellulose coat,
becomes globular and passes into a quiescent state again.
Much of this part of its history is common to lowly animal
forms.
Conclusions. — It will be seen that there is much in common
in the life-history of Torula and Protococcus. By virtue of
being living protoplasm they transform unorganized material
into their own substance ; and they grow and reproduce by
analogous methods.
But there are sharply denned differences. For the green
plant sunlight is essential, in the presence of which its chloro-
phyl prepares the atmosphere for animals by the removal of
carbonic anhydride and the addition of oxygen, while for
Torula neither this gas nor sunlight is essential.
Moreover, the fungus (Torula) demands a higher kind of
food, one more nearly related to the pabulum of animals ; and
is absolutely independent of sunlight, if not actually injured by
it ; not to mention the remarkable process of fermentation.
GENERAL BIOLOGY. 13
UNICELLULAR ANIMALS.
The Proteus Animalcule (Amoeba).
In order to illustrate animal life in its simpler form we
choose the above-named creature, which is nearly as readily
obtainable as Protococcus and often under the same circum-
stances.
Morphological. — Amoeba is a microscopic mass of transpar-
ent protoplasm, about the size of the largest of the colorless
blood-corpuscles of cold-blooded animals, with a clearer, more
consistent outer zone (ectosarc), (although without any proper
cell wall), and a more fluid, granular inner part. A clear space
(contractile vesicle, vacuole) makes its appearance at intervals
in the ectosarc, which may disappear somewhat suddenly. This
appearance and vanishing have suggested the term pulsating
or contracting vesicle. Both a nucleus and nucleolus may be
seen in Amoeba. At varying short periods certain parts of its
body ( pseudopodia) are thrust out and others withdrawn.
Physiological. — Amoeba can not live on such food as proves
adequate for either Protococcus or Torula, but requires, besides
inorganic and unorganized food, also organized matter in the
form of a complex organic compound known as protein, which
contains nitrogen in addition to carbon, hydrogen, and oxygen.
In fact, Amoeba can prey upon both plants and animals, and
thus use up as food protoplasm itself. The pseudopodia serve
the double purpose of organs of locomotion and prehension.
This creature absorbs oxygen and evolves carbon dioxide.
Inasmuch as any part of the body may serve for the admission,
and possibly the digestion, of food and the ejection of the use-
less remains, we are not able to define the functions of special
parts. Amoeba exercises, however, some degree of choice as to
what it accepts or rejects.
The movements of the pseudopodia cease when the tempera-
ture of the surrounding medium is raised or lowered beyond a
certain point. It can, however, survive in a quiescent form
greater depression than elevation of the temperature. Thus, at
35° C, heat-rigor is induced; at 40° to 45° C, death results ;
but though all movement is arrested at the freezing-point of
water, recovery ensues if the temperature be gradually raised.
Its form is modified by electric shocks and chemical agents,
as well as by variations in the temperature. At the pres-
ent time it is not possible to define accurately the functions
14
COMPARATIVE PHYSIOLOGY.
of the vacuoles found in any of the organisms thus far consid-
ered. It is worthy of note that Amoeba may spontaneously
assume a spherical form, secrete a structureless covering, and
Fig. 14.
Fig. 15.
Fig. 10.
Figs. 8 to 15, represent successive phases in the life-history of an Amoeboid organism-
kept under constant observation for three days ; Fig. 16 a similar organism en"
cysted, which was a few hours later set free by the disintegration of the cyst.
(All the figures arc drawn under Zeiss, D. 3.)
Fra. 8. — The locomotor phase ; the ectoplasm is seen protruding to form a pseudopo-
(liuiri, into which the endoplasm passes.
Fig. 9. — A stage in the ingestive phase. A vegetable organism,,/)?, is undergoing in-
tussusception.
Fig. 10.— A portion of the creature represented in Fig. 9, after complete ingestion of
the food-particle.
Fig. 11, 12. — Successive stages in the assimilative and excretory processes. Fig. 12
represents the organism some twenty hours later than as seen in Fig. 11. The
undigested remnants of the ingested organism are represented undergoing ejec-
tion (excretion) atfp, in Fig. 12.
Figs. 13, 14, 15, represent successive stages in the reproductive process of the same in-
dividual, observed two days later. It will be noticed (Fig. 13) that the nucleus di-
vides first.
In tin: above figures, vc, denotes the contracting vacuole ; nc, the nucleus ; ps, pseu-
dopodium ; M, diatom ; fp, food-particle.
GENERAL BIOLOGY. 15
remain in this condition for a variable period, reminding us of
the similar behavior of Torula.
Amoeba reproduces by fission, in which the nucleus takes a
prominent if not a directive part, as seems likely in regard to
all the functions of unicellular organisms.
Conclusions. — It is evident that Amoeba is, in much of its
behavior, closely related to both colored arid colorless one-celled
plants. All of the three classes of organisms are composed of
protoplasm ; each can construct protoplasm out of that which
is very different from it ; each builds up the inanimate inor-
ganic world into itself by virtue of that force which we call
vital, but which in its essence we do not understand ; each mul-
tiplies by division of itself, and all can only live, move, and
have their being under certain definite limitations. But even
among forms of life so lowly as those we have been consider-
ing, the differences between the animal and vegetable worlds
appear. Thus, Amoeba never has a cellulose wall, and can not
subsist on inorganic food alone. The cellulose wall is not, how-
ever, invariably present in plants, though this is generally the
case ; and there are animals (Ascidians) with a cellulose invest-
ment. Such are very exceptional cases. But the law that ani-
mals must have organized material (protein) as food is without
exception, and forms a broad line of distinction between the
animal and vegetable kingdoms.
Amoeba will receive further consideration later ; in the
mean time, we turn to the study of forms of life in many re-
spects intermediate between plants and animals, and full of prac-
tical interest for mankind, on account of their relations to dis-
ease, as revealed by recent investigations.
PARASITIC ORGANISMS.
The Fungi.
Molds (Penicillmm glaucum and Mucor mucedo).
Closely related to Torula physiologically, but of more com-
plex structure, are the molds, of which we select for convenient
study the common green mold (Penieilliiim), found growing in
dark and moist places on bread and similar substances, and the
white mold {Mucor), which grows readily on manure.
The fungi originate in spores, which are essentially like
Torula in structure, by a process of budding and longitudinal
extension, resulting in the formation of transparent branches
16
COMPARATIVE PHYSIOLOGY.
3«PP
GENERAL BIOLOGY. 17
Pigs. 17 to 28. — In the following figures, ha, denotes ae>ial hyphse; sp, sporangium;
zy, sygospore; ex, exosporium; my, mycelium; mc, mucilage; cl, columella; en,
endogonidia.
Fro. 17.— Spore-bearin<* hyphse of Mucor, growing from horse-dung.
Fti*. 1 .—The same, teased out with needles (A, 4).
Figs. 19, 20, 21.— Successive stages in the development of the sporangium.
Fig. 22.— Isolated spores of Mucor.
Fig. 23. — Germinating spores of the same mold.
Fig. 24.— Successive stages in the germination of a single spore.
Figs. 25, 26, 27.— Successive phases in the conjugative process of Mucor.
FiG. 28.— Successive stages observed during ten hours in the growth of a conidiophore
of Penicillium in an object-glass culture (D, 4).
or tubules, filled with protoplasm and invested by cellulose
walls, across which transverse partitions are found at regular
intervals, and in which vacuoles are also visible.
The spores, when growing thus in a liquid, gives rise to up-
ward branches (aerial hyphce), and downward branches or root-
lets (submerged hyphce). These multitudinous branches inter-
lace in every direction, forming an intricate felt-work, which
supports the green powder (spores) which may be so easily
shaken off from a growing mold. In certain cases the aerial
hyphae terminate in tufts of branches, which, by transverse
division, become split up into spores (Conidia), each of which
is similar in structure to a yeast-cell.
The green coloring matter of the fungi is not chlorophyl.
The Conidia germinate under the same conditions as Torula.
Mucor mucedo. — The growth and development of this mold
may be studied by simply inverting a glass tumbler over some
horse-dung on a saucer, into which a very little water has been
poured, and keeping the preparation in a warm place.
Very soon whitish filaments, gradually getting stronger, ap-
pear, and are finally topped by rounded heads or spore-cases
{Sporangia). These filaments are the hyphce, similar in struct-
ure to those of Penicillium. The spore-case is filled with a
multitude of oval bodies (spores), resulting from the subdivision
of the protoplasm, which are finally released by the spore-case
becoming thinned to the point of rupture. The development
of these spores take place in substantially the same manner as
those of Penicillium. Sporangia developing spores in this fash-
ion by division of the protoplasm are termed asci, and the spores
ascospores.
So long as nourishment is abundant and the medium of
growth fluid, this asexual method of reproduction is the only
one ; but, under other circumstances, a mode of increase, known
as conjugation, arises. Two adjacent hyphse enlarge at the ex-
tremities into somewhat globular heads, bend over toward each
2
18 COMPARATIVE PHYSIOLOGY.
other, and, meeting, their opposed faces hecome thinned, and
the contents intermingle. The result of this union {zygospore)
undergoes now certain further changes, the cellulose coat heing
separated into two — an outer, darker in color (exosporium),
and an inner colorless one (endosporium) .
Under favoring circumstances these coats burst, and a
branch sprouts forth from which a vertical tube arises that
terminates in a sporangium, in which spores arise, as before de-
scribed. It will be apparent that we have in Mucor the exem-
plification of what is known in biology as " alternation of gen-
erations ".—that is, there is an intermediate generation be
tween the original form and that in which the original is
again*reached.
Physiologically the molds closely resemble yeast, some of
them, as Mucor, being capable of exciting a fermentation.
The fungi are of special interest to the medical student, be-
cause many forms of cutaneous disease are directly associated
with their growth in the epithelium of the skin, as, for exam-
ple, common ringworm ; and their great vitality, and the facil-
ity with which their spores are widely dispersed, explain the
highly contagious nature of such diseases. The media on which
they flourish (feed) indicates their great physiological differ-
ences in this particular from the green plants proper. They are
closely related in not a few respects to an important class of
vegetable organisms, known as bacteria, to be considered forth-
with.
The Bacteria.
The bacteria include numberless varieties of organisms of
extreme minuteness, many of them visible only by the help of
the most powerful lenses. Then size has been estimated at
from ^xiiny to i0^6o of an inch in diameter.
They grow mostly in the longitudinal direction, and repro-
duce by transverse division, forming spores from which new
generations arise.
Some of them have vibratile cilia, while the cause of the
movements of others is quite unknown.
As in many other lowly forms of life, there is a quiescent
as well as an active stage. In this stage (zoogloea form) they
are surrounded by a gelatinous matter, probably secreted by
themselves.
Bacteria grow and reproduce in Pasteur's solution, rendering
it opaque, as well as in almost all fluids that abound in proteid
GENERAL BIOLOGY.
19
matter. That such fluids readily putrefy is owing to the pres-
ence of bacteria, the vital action of which suffices to break asun-
Fig. 89.
Fig. 32
Fig. 29.— Micrococcus, very like a spore, but usually much smaller.
Fig. 30.— Bacterium.
Fig. 31.— Bacillus. The central filament presented this segmented appearance as the
result of a process of transverse division occurring during ten minutes' obser-
vation.
Fig. 32.— Spirillum; various forms. The first two represent vibrio, which is possibly
only a stage of spirillum.
Fig. 33. — A drop of the surface scum, showing a spirillum aggregate in the resting
state.
der complex chemical compounds and produce new ones. Some
of the bacteria require oxygen, as Bacillus anthracis, while
others do not, as the organism of putrefaction, Bacterium
termo.
Bacteria are not so sensitive to slight variations in tempera-
ture as most other organisms. They can, many of them, with-
stand freezing and high temperatures. All bacteria and all
germs of bacteria are killed by boiling water, though the spores
20 COMPARATIVE PHYSIOLOGY.
are much more resistant than the mature organisms themselves.
Some spores can resist a dry heat of 140° C.
The spores, like Torula and Protococcus, bear drying, with-
out loss of vitality, for considerable periods.
That different groups of bacteria have a somewhat different
life-history is evident from tbe fact that the presence of one
checks the other in the same fluid, and that successive swarms
of different kinds may flourish where others have ceased to
live.
That these organisms are enemies of the constituent cells of
the tissues of the highest mammals has now been abundantly
demonstrated. That they interfere with the normal working
of the organism in a great variety of ways is also clear ; and
certain it is that the harm they do leads to aberration in cell-
life, however that may be manifested. They rob the tissues of
their nutriment and oxygen, and poison them by the products
of the decompositions they produce. But apart from this, their
very presence as foreign agents must hamper and derange the
delicate mechanism of cell-life.
These organisms seem to people the air, land, and waters
with invisible hosts far more numerous than the forms of life
we behold. Fortunately, they are not all dangerous to the
higher forms of mammalian life ; but that a large proportion
of the diseases which afflict both man and the domestic animals
are directly caused by the presence of such forms of life, in the
sense of being invariably associated with them, is now beyond
doubt.
The facts stated above explain why that should be so ; why
certain maladies should be infectious ; how the germs of dis
ease may be transported to a friend wrapped up in the folds of
a letter.
Disease thus caused, it must not be forgotten, is an illustra-
tion of the struggle for existence and the survival of the fittest.
If the cells of an organism are mightier than the bacteria, the
latter are overwhelmed ; but if the bacteria are too great in
numbers or more vigorous, the cells must yield ; the battle may
waver — now dangerous disease, now improvement — but in the
end the strongest in this, as in other instances, prevail.
GENERAL BIOLOGY. 21
UNICELLULAR ANIMALS WITH DIFFERENTIATION
OF STRUCTURE.
The Bell- Animalcule (Vorticella).
Amoeba is an example of a one-celled animal with little per-
ceptible differentiation of structure or corresponding- division
of physiological labor. This is not, however, the case with all
unicellular animals, and we proceed to study one of these with
considerable development of -both. The Bell - animalcule is
found in both fresh and salt water, either single or in groups.
It is anchored to some object by a rope-like stalk of clear pro-
toplasm, that has a spiral appearance when contracted ; and
which, with a certain degree of regularity, shortens and length
ens alternately, suggesting that more definite movement (con-
traction) of the form of protoplasm known as muscle, to be
studied later.
The body of the creature is bell-shaped, hence its name ; the
bell being provided with a thick everted lip (peristome), covered
with bristle-like extensions of the protoplasm (cilia), which are
in almost constant rhythmical motion. Covering the mouth of
the bell is a lid, attached by a hinge of protoplasm to the body,
which may be raised or lowered A wide, funnel-like depres-
sion {oesophagus) leads into the softer substance within which
it ends blindly. The outer part of the animal (cuticula) is
denser and more transparent than any other part of the whole
creature ; next to this is a portion more granular and of inter-
mediate transparency between the external and innermost por-
tions (cortical layer). Below the disk is a space (contractile
vesicle) filled with a thin, clear fluid, which may be seen to en-
large slowly, and then to collapse suddenly. When the Vorti-
cella is feeding, these vesicles may contain food-particles, and
in the former, apparently, digestion goes on. Such food vacu-
oles (vesicles) may circulate up one side of the body of the ani-
mal and down the other. Their exact significance is not known,
but it would appear as if digestion went on within them ; and
possibly the clear fluid with which they are filled may be a spe-
cial secretion with solvent action on food.
Situated somewhat centrally is a horseshoe-shaped body, with
well-defined edges, which stains more readily than the rest of the
cell, indicating a different chemical composition ; and, from the
prominent part it takes in the reproductive and other functions
of the creature, it may be considered the nucleus (endoplast).
22
COMPARATIVE PHYSIOLOGY.
Multiplication of the species is either by gemmation or by
fission. In the first case the nucleus divides and the frag-
Fig. 37.
Fig. 40.
Fig. 38.
Fig. 39.
Figs. 34 to 40.— In the figures d denotes disk ; p,
peristome; vc, contractile vacuole; vf, food-
vacuole; vs, vestibule; cf, contractile fiber;
c, cyst; nc, nucleus; cl, c'ilium.
Fig. 34. — A group of vorticellte showing the crea-
ture in various positions (A, 3).
Fig. 35. — The same, in the extended and in the
retracted state. (Surface views.)
Fig. 36.— Shows food-vacuoles; one in the act of
ingestion.
Fig. 37. — A vorticella, in which the process of
multiplication by fission is begun.
Fig. 38. — The results of fission; the production
of two individuals of unequal size.
Fig. 39.— Illustration of reproduction by conju-
gation.
Fig. 40.— An encysted vorticella.
Fig. 35,
ments are transformed into locomotive germs; in the latter
the entire animal, including the nucleus, divides longitudi-
nally, each half becoming a similar complete, independent or-
ganism. Still another method of reproduction is known. A
more or less globular body encircled with a ring of cilia and
of relatively small size may sometimes be seen attached to
the usual form of Vorticella, with which it finally becomes
blended into one mass. This seems to foreshadow the " sexual
GENERAL BIOLOGY. 23
conjugation " of higher forms, and is of great biological sig-
nificance.
Vorticella may pass into an encysted and quiescent stage for
an indefinite period and again become active. The history of
the Bell-animalcule is substantially that of a vast variety of
one-celled organisms known as Infusoria, to which Amoeba
itself belongs. It will be observed that the resemblance of this
organism to Amoeba is very great; it is, however, introduced
here to illustrate an advance in differentiation of structure ; and
to show how, with the latter, there is usually a physiological
advance also, since there is additional functional progress or
division of labor ; but still the whole of the work is done with-
in one cell. Amoeba and Vorticella are both factories in which
all of the work is done in one room, but in the latter case the
machinery is more complex than in the former ; there are cor-
respondingly more processes, and each is performed with greater
perfection. Thus, food in the case of the Bell-animalcule is
swept into the gullet by the currents set up by the multitudes
of vibrating arms around this opening and its immediate neigh-
borhood ; the contractile vesicles play a more prominent part ;
and the waste of undigested food is ejected at a more definite
portion of the body, the floor of the oesophagus ; while all the
movements of the animal are rhythmical to a degree not exem-
plified in such simple forms as Amoeba; not to mention its
various resources for multiplication and, therefore, for its
perpetuation and permanence as a species. It, too, like all the
unicellular organisms we have been considering, is susceptible
of very wide distribution, being capable of retaining vitality in
the driedf state, so that these infusoria may be carried in vari-
ous directions by winds in the form of microscopic dust.
MULTICELLULAR ORGANISMS.
The Fresh- Water Polyps {Hydra viridis ; Hydra fused).
The comparison of an animal so simple in structure, though
made up of many cells, as the Polyp, with the more complex
organizations with which we shall have especially to deal, may
be fitly undertaken at this stage. The Polyps are easily obtain-
able from ponds in which they are found attached to various
kinds of weeds. To the naked eye, they resemble translucent
masses of jelly with a greenish or reddish tinge. They range
in size from one quarter to one half an inch ; are of an elongated
COMPARATIVE PHYSIOLOGY.
Fio. 40,
GENERAL BIOLOGY. 25
Figs. 41 to 46.— In the figures ec denotes ectoderm; en, endoderm.; /, tentacle; hp,
hypostome; /, foot; te, testes; ov, ovary; ps, pseudopodiam; ec', larger ectoderm
cells: ne\ larger nematocysts before rupture; cp, Kleinenberg's fibers; <■./. sup-
porting lamella; c/„ chlorophyl-forruing bodies; e, cilium.
Fig. 41. — The green hydra, at the maximum of contraction and elongation of its body.
The creature is represented in the act of seizing a small crustacean (A, 2).
Fig. 42. — Transverse section across the body of a hydra, in the digestive cavity of
which a small crustacean is represented.
Fig. 43.— The leading types of thread-cells, after liberation from the body (P, 3). The
cells are represented in the active and the resting conditions; in the former all the
parts are more distinctly seen in consequence of the necessary eversion.
Fig. 44.— Small portion of a transverse section across the bodv of a green hydra
(D, 3).
Fig. 45. — A large brown hydra bearing at the same time buds produced asexually and
sexual organs.
Fig. 4tj. — Larger cells of the ectoderm isolated. Note the processes of the cells or
Kleinenberg's fibers (F, 3).
All the cuts on pages 9 to 34 have been selected from Howes' Atlas of Biology.
cylindrical form ; provided at the oral extremity with thread-
like tenacles of considerable length, which are slowly moved
about in all directions ; but they and the entire body may short-
en rapidly into a globular mass. They are usually attached at
the opposite (aboral) pole to some object, but may float free, or
slowly crawl from place to place. It may be observed, under
the microscope, that the tenacles now and then embrace some
living object, convey it toward an opening (mouth) near their
base, from which, from time to time, refuse material is cast out.
It may be noticed, too, that a living object within the touch of
these tenacles soon loses the power to struggle, which is owing
to the peculiar cells (nettle-cells, urticating capsules, nemato-
cysts) with which they are abundantly provided, and which se-
crete a poisonovis fluid that paralyzes prey.
The mouth leads into a simple cavity (coelom) in which
digestion proceeds. The green color in Hydra viridis, and the
red color of Hydra fusca, is owing to the presence of chlorophyl,
the function of which is not known. Hydra is structurally a
sac, made up of two layers of cells, an outer (ectoderm) and
an inner (endoderm): the tentacles being repetitions of the
scructure of the main body of the animal, and so hollow and
composed of two cell layers. Speaking generally, the outer
layer is devoted to obtaining information of the surroundings ;
the inner to the work of preparing nutriment, and probably,
also, discharging waste matters, in which latter assistance is
also received from the outer layer. As digestion takes place
largely within the cells themselves, or is intracellular, we are
reminded of Vorticella and still more of Amoeba. There is in
Hydra a general advance in development, but not very much
individual cell specialization. That of the urticating capsules is
one of the best examples of such specialization in this creature.
26 COMPARATIVE PHYSIOLOGY.
A Polyp is like a colony of Amoebae in which some division of
labor (function) has taken place ; a sort of biological state in
which every individual is nearly equal to his neighbor, but
somewhat more advanced than those neighbors not members of
the organization.
But in one respect the Polyps show an enormous advance.
Ordinarily when nourishment is abundant Hydra multiplies by
budding, and when cut into portions each may become a com-
plete individual. However, under other circumstances, near
the bases of the tentacles the body wall may protrude into little
masses (tes es), in which cells of peculiar formation (sperma-
tozoa) arise, and are eventually set free and unite with a cell-
{ovum) formed in a similar protrusion of larger size (ovary).
Here, then, is the first instance in which distinctly sexual repro-
duction has been met in our studies of the lower forms of life.
This is substantially the same process in Hydra as in mammals.
But, as both male and female cells are produced by the same
individual, the sexes are united (hermaphroditism) ; each is at
once male and female.
Any one watching the movements of a Polyp, and compar-
ing it with those of a Bell-animalcule, will observe that the
former are much less machine-like ; have greater range ; seem
to be the result of a more deliberate choice ; are better adapted
to the environment, and calculated to achieve higher ends. In
the absence of a nervous system it is not easy to explain how
one part moves in harmony with another, except by that pro-
cess which seems to be of such wide application in nature, adap-
tation from habitual simultaneous effects on a protoplasm capa-
ble of responding to stimuli. When one process of an Amoeba
is touched, it is likely to withdraw all. This we take to be due
to influences radiating through molecular movement to other
parts ; the same principle of action may be extended to Hydra.
The oftener any molecular movement is repeated, the more it
tends to become organized into regularity, to become fixed in
its mode of action ; and if we are not mistaken this is a funda-
mental law throughout the entire world of living things, if not
of all things animate and inanimate alike. To this law we
shall return.
But Hydra is a creature of but very limited specializations;
there are neither organs of circulation, respiration, nor excretion,
if we exclude the doubtful case of the thread-cells (urticating
capsules). The animal breathes by the entire surface of the
GENERAL BIOLOGY. 27
body ; nourishment passes from cell to cell, and waste is dis-
charged into the water surrounding the creature from all cells,
though probably not quite equally. All parts are not digestive,
respiratory, etc., to the same degree, and herein does it differ
greatly from Amoeba or even Vorticella, though fuller knowl-
edge will likely modify our views of the latter two and similar
organisms in this regard.
THE CELL RECONSIDERED.
Having now studied certain one-celled plants and animals,
and some very simple combinations of cells (molds, etc.), it will
be profitable to endeavor to generalize the lessons these humble
organisms convey ; for, as will be constantly seen in the study of
the higher forms of life of which this work proposes to treat
principally, the same laws operate as in the lowliest living creat-
ures. The most complex organism is made up of tissues, which
are but cells and then* products, as houses are made of bricks,
mortar, wood, and a few other materials, however large or elab-
orate.
The student of physiology who proceeds scientifically must
endeavor, in investigating the functions of each organ, to learn
the exact behavior of each cell as determined by its own inherent
tendencies, and modified by the action of neighboring cells.
The reason why the function of one organ differs from that of
another is that its cells have departed in a special direction from
those properties common to all cells, or have become function-
ally differentiated. But such a statement has no meaning un-
less it be well understood that cells have certain properties in
common. This is one of the lessons imparted by the preceding
studies which we now review. Briefly stated in language now
extensively used in works on biology, the common properties of
cells' (protoplasm), whether animal or vegetable, whether consti-
tuting in themselves entire animals or plants, or forming the
elements of tissues, are these : The collective chemical processes
associated with the vital activities of cells are termed its metab-
olism. Metabolism is constructive when more complex com-
pounds are formed from simple ones, as when the Protococcus-
cell builds up its protoplasm out of the simple materials, found in
rain-water, which makes up its food. Metabolism, is destructive
when the reverse process takes place. The results of this process
are eliminated as excreta, or useless and harmful products.
28 COMPARATIVE PHYSIOLOGY.
Since all the vital activities of cells can only be manifested when
supplied with food, it follows that living organisms convert po-
tential or possible energy into kinetic or actual energy. When
lifeless, immobile matter is taken in as food and, as a result, is
converted by a process of assimilation into the protoplasm of the
cell using it, we have an example of potential being converted
into actual energy, for one of the properties of all protoplasm is
its contractility. Assimilation implies, of course, the absorp-
tion of what is to be used, with rejection of waste matters.
The movements of protoplasm of whatever kind, when due
to a stimulus, are said to indicate irritability ; while, if inde-
pendent of any external source of excitation, they are denomi-
nated automatic.
Among agents that modify the action of all kinds of proto-
plasm are heat, moisture, electricity, light, and others in great
variety, both chemical and mechanical. It can not be too well
remembered that living things are what they are, neither by
virtue of their own organization alone nor through the action
of their environment alone (else would they be in no sense dif-
ferent from inanimate things), but because of the relation of
the organization to the surroundings.
Protoplasm, then, is contractile, irritable, automatic, absorp-
tive, secretory (and excretory), metabolic, and reproductive.
But when it is affirmed that these are the fundamental prop-
erties of all protoplasm, the idea is not to be conveyed that cells
exhibiting these properties are identical biologically. No two
masses of protoplasm can be quite alike, else would there be no
distinction in physiological demeanor — no individuality. Every
cell, could we but behold its inner molecular mechanism, differs
from its neighbor. When this difference reaches a certain de-
gree in one direction, we have a manifest differentiation leading
to physiological division of labor, which may now with advan-
tage be treated in the following section.
THE ANIMAL BODY.
An animal, as we have learned, may be made up of a single
cell in which each part performs much the same work ; or, if
there be differences in function, they are ill-defined as compared
with those of higher animals. The condition of tilings in such
an animal as Amoeba may be compared to a civilized commu-
nity in a very crude social condition. When each individual
GENERAL BIOLOGY. 29
tries to perform every office for himself, he is at once carpenter,
blacksmith, shoemaker, and much more, with the natural re-
sult that he is not efficient in any one direction. A community
may be judged in regard to its degree of advancement by the
amount of division of labor existing within it. Thus is it with
the animal body. We find in such a creature as the fresh-water
Hydra, consisting of two layers of cells forming a simple sac, a
slight amount of advancement on Amoeba. Its external surface
no longer serves for inclosure of food, but it has the simplest
form of mouth and tentacles. Each of the cells of the internal
layer seems to act as a somewhat improved or specialized Amoe-
ba, while in those of the outer layer we mark a beginning of
those functions which taken collectively give the higher ani-
mals information of the surrounding world.
Looking to the existing state of things in the universe, it is
plain that an animal to attain to high ends must have powers
of rapid locomotion, capacity to perceive what makes for its in-
terest, and ability to utilize means to obtain this when perceived.
These considerations demand that an animal high in the scale
of being should be provided with limbs sufficiently rigid to sup-
port its weight, moved by strong muscles, which must act in
harmony. But this implies abundance of nutriment duly pre-
pared and regularly conveyed to the bones and muscles. All
this would be useless unless there was a controlling and ener-
gizing system capable both of being impressed and originating
impressions. Such is found in the nerves and nerve- centers.
Again, in order that this mechanism be kept in good running
order, the waste of its own metabolism, which chokes and poi-
sons, must be got rid of— hence the need of excretory apparatus.
In order that the nervous system may get sufficient informa-
tion of the world around, the surface of the body must be pro-
vided with special message-receiving offices in the form of
modified nerve-endings. In short, it is seen that an animal as
high in the scale as a mammal must have muscular, osseous
(aud connective), digestive, circulatory, excretory, and nervous
tissues ; and to these may be added certain forms of protective
tissues, as hair, nails, etc. *
Assuming that the student has at least some general knowl-
edge of the structure of these various tissues, we propose to tell
in a simple way the whole physiological story in brief.
The blood is the source of all the nourishment of the organ-
ism, including its oxygen supply, and is carried to eveiy part of
30 COMPARATIVE PHYSIOLOGY.
the body through elastic tubes which, continually branching
and becoming gradually smaller, terminate in vessels of hair-
like fineness in which the current is very slow — a condition per-
mitting that interchange between the cells surrounding them
and the blood which may be compared to a process of barter,
the cells taking nutriment and oxygen, and giving (excreting)
in return carbonic anhydride. From these minute vessels the
blood is conveyed back toward the source whence it came by
similar elastic tubes which gradually increase in size and be-
come fewer. The force which directly propels the blood in its
onward course is a muscular pump, with both a forcing and
suction action, though chiefly the former. The flow of blood
is maintained constant owing to the resistance in the smaller
tubes on the one hand and the elastic recoil of the larger tubes
on the other ; while in the returning vessels the column of
blood is suppoi'ted by elastic double gates which so close as to
prevent reflux. The oxygen of the blood is carried in disks of
microscopic size which give it up in proportion to the needs of
the tissues past which they are cai^ried.
But in reality the tissues of the body are not nourished
directly by the blood, but by a fluid derived from it and resem-
bling it greatly in most particulars. This fluid bathes the tis-
sue-cells on all sides. It also is taken up by tubes that convey
it into the blood after it has passed through little factories
(lymphatic glands), in which it undergoes a regeneration.
Since the tissues are impoverishing the blood by withdrawal of
its constituents, and adding to it what is no longer useful, and
is in reality poisonous, it becomes necessary that new material
be added to it and the injurious components withdrawn. The
former is accomplished by the absorption of the products of
food digestion, and the addition of a fresh supply of oxygen
derived from without, while the poisonous ingredients that
have found their way into the blood are got rid of through
processes that may be, in general, compared to those of a sew-
age system of a very elaborate character. To explain this re-
generation of the blood in somewhat more detail, we must first
consider the fate of food from the time it enters the mouth till
it leaves the tract of the body in which its preparation is car-
ried on.
The food is in the mouth submitted to the action of a series
of cutting and grinding organs worked by powerful muscles ;
mixed with a fluid which changes the starchy part of it into
GENERAL BIOLOGY. 31
sugar, and prepares the whole to pass further on its course :
when this has heen accomplished, the food is grasped and
squeezed and pushed along the tube, owing to the action of its
own muscular cells, into a sac (stomach), in which it is rolled
about and mixed with certain fluids of peculiar chemical com-
position derived from cells on its inner surface, which trans-
form the proteid part of the food into a form susceptible of
ready use (absorption). When this saccular organ has done
its share of the work, the food is moved on by the action of
the muscles of its walls into a very long portion of the tract
in which, in addition to processes carried on in the mouth and
stomach, there are others which transform the food into a con-
dition in which it can pass into the blood. Thus, all of the
food that is susceptible of changes of the kind described is acted
upon somewhere in the long tract devoted to this task. But
there is usually a remnant of indigestible material which is
finally evacuated. How is the prepared material conveyed into
the blood ? In part, directly through the walls of the minutest
blood-vessels distributed throughout the length of this tube ;
and in part through special vessels with appropriate cells cov-
ering them which act as minute porters (villi).
The impure blood is carried periodically to an extensive sur-
face, usually much folded, and there exposed in the hair-like
tubes referred to before, and thus parts with its excess of car-
bon dioxide and takes up fresh oxygen. But all the functions
described do not go on in a fixed and invariable manner, but
are modified somewhat according to circumstances. The for-
cing-pump of the circulatory system does not always beat
equally fast ; the smaller blood-vessels are not always of the
same size, but admit more or less blood to an organ according
to its needs.
This is all accomplished in obedience to the commands car-
ried from the brain and spinal cord along the nerves. All
movements of the limbs and other parts are executed in obe-
dience to its behests ; and in order that these may be in accord-
ance with the best interests of each particular organ and the
whole animal, the nervous centers. Which may be compared to
the chief officers of, say, a telegraph or railway system, are in
constant receipt of information by messages carried onward
along the nerves. The command issuing is always related to
the information arriving.
All those parts commonly known as sense-organs — the eye,
32 COMPARATIVE PHYSIOLOGY.
ear, nose, tongue, and the entire surface of the body — are faith-
ful reporters of facts. They put the inner and outer worlds in
communication, and without them all higher life at least must
cease, for the organism, like a train directed by a conductor that
disregards the danger-signals, must work its own destruction.
Without going into further details, suffice it to say that the pro-
cesses of the various cells are subordinated to the general good
through the nervous system, and that susceptibility of proto-
plasm to stimuli of a delicate kind which enables each cell to
adapt to its surroundings, including the influence of remote as
well as neighboring cells. Without this there could be no
marked advance in organisms, no differentiation of a pro-
nounced character, and so none of that physiological division
of labor which will be inferred from our brief description of
the functions of a mammal. The whole of physiology but
illustrates this division of labor.
It is hoped that the above account of the working of the ani-
mal body, brief as it is, may serve to show the connection of
one part functionally with another, for it is much more impor-
tant that this should be kept in mind throughout, than that all
the details of any one function should be known.
LIVING AND LIFELESS MATTER.
In order to enable the student the better to realize the na-
ture of living matter or protoplasm, and to render clearer the
distinction between the forms that belong to the organic and
inorganic worlds respectively, we shall make some comparisons
in detail which it is hoped may accomplish this object.
A modern watch that keeps correct time must be regarded
as a wonderful object, a marvelous triumph of human skill.
That it has aroused the awe of savages, and been mistaken for a
living being, is not surprising. But, admirable as is the result
attained by the mechanism of a watch, it is, after all, composed
of but a few metals, etc., chiefly in fact of two, brass and steel ;
these are, however, made up into a great number of different
parts, so adapted to one another as to work in unison and ac-
complish the desired object of indicating the time of day.
Now, however well constructed the watch may be, there are
waste, wear and tear, which will manifest themselves more and
more, until finally the machine becomes worthless for the pur-
pose of its construction. If this mechanism possessed the power
GENERAL BIOLOGY. 33
of adapting- from without foreign matter so as to construct it
it into steel and brass, and arrange this just when required, it
would imitate a living organism ; but this it can not do, nor is
its waste chemically different from its component metals ; it
does not break up brass and steel into something wholly differ-
ent. In one particular it does closely resemble living things,
in that it gradually deteriorates ; but the degradation of a liv-
ing cell is tbe consequence of an actual change in its compo-
nent parts, commonly a fatty degeneration. The one is a real
transformation, the other mere wear.
Had the watch the power to give rise to a new one like itself
by any process, especially a process of division of itself into two
parts, we should have a parallel with living forms ; but the
watch can not even renew its own parts, much less give rise to
a second mechanism like itself. Here, then, is a manifest dis-
tinction between living and inanimate things.
Suppose, further, that the watch was so constructed that,
after the lapse of a certain time, it underwent a change in its
inner machinery and perhaps its outer form, so as to be scarcely
recognizable as the same ; and that as a result, instead of indi-
cating the hours and minutes of a time-reckoning adapted to
the inhabitants of our globe, it indicated time in a wholly dif-
ferent way ; that after a series of such transformations it fell to
pieces— took the original form of the metals from which it was
constructed — we should then have in this succession of events a
parallel with the development, decline, and death of living or-
ganisms.
In another particular our illustration of a watch may serve
a useful purpose. Suppose a watch to exist, the works of which
are so concealed as to be quite inaccessible to our vision, so that
all we know of it is that it has a mechanism which when in
action we can hear, and the result of which we perceive in the
movements of the hands on the face ; we should then be in the
exact position in reference to the watch that we now are as re-
gards the molecular movements of protoplasm. On the latter
the entire behavior of living matter depends ; yet it is abso-
lutely hidden from us.
We know, too, that variations must be produced in the
mechanism of time-pieces by temperature, moisture, and other
influences of the environment, resulting in altered action. The
same, as will be shown in later chapters, occurs in protoplasm.
Tins, too, is primarily a molecular effect. If the works of
3
34 COMPARATIVE PHYSIOLOGY.
watches were beyond observation, we should not be able to state
exactly how the variations observed in different kinds, or even
different individuals of the same kind occurred, though these
differences might be of the most marked character, such as any
one could recognize. Here once more we refer the differ-
ences to the mechanism. So is it with living beings : the ulti-
mate molecular mechanism is unknown to us.
Could we but render these molecular movements visible to
our eyes, we should have a revelation of far greater scientific
importance than that unfolded by the recent researches into
those living forms of extreme minuteness that swarm every-
where as dust in a sunbeam, and, as will be learned later, are
often the source of deadly disease. Like the movements of the
watch, the activities of protoplasm are ceaseless. A watch that
will not run is, as such, worthless — it is mere metal — has under-
gone an immense degradation in the scale of values ; so proto-
plasm is no longer protoplasm when its peculiar molecular
movements cease ; it is at once degraded to the rank of dead
matter.
The student may observe that each of the four propositions,
embodying the fundamental properties of living matter, stated
in the preceding chapter, have been illustrated by the simile of
a watch. Such an illustration is necessarily crude, but it helps
one to realize the meaning of truths which gather force with
each living form studied if regarded aright ; and it is upon the
realization of truth that mental growth as well as practical
efficiency depends.
CLASSIFICATION OF THE ANIMAL KINGDOM.
There are human beings so low in the scale as not to possess
such general terms as tree, while they do employ names for dif-
ferent kinds of trees. The use of such a word as " tree " im-
plies generalization, or the abstraction of a set of qualities from
the things in which they reside, and making them the basis for
the grouping of a multitude of objects by which we are sur-
rounded. Manifestly without such a process knowledge must
be very limited, and the world without significance ; while in
proportion as generalization may be safely widened, is our
progress in the unification of knowledge toward which science
is tending. But it also follows that without complete knowl-
edge there can be no perfect classification of objects ; hence,
GENERAL BIOLOGY. 35
any classification must be regarded but as the temporary creed
of science, to be modified with the extension of knowledge. As
a matter of fact this has been the history of all zoological and
other systems of arrangement. The only purpose of grouping
is to simplify and extend knowledge ; this being the case, it fol-
lows that a method of grouping that accomplishes this has
value, though the system may be artificial that is based on
resemblances which, though real and constant, are associated
with differences so numerous and radical that the total amount
of likeness between objects thus grouped is often less than the
difference. Such a system was that of Linnaeus, who classified
plants according to the number of stamens, etc., they bore.
Seeing that animals which resemble each other are of com-
mon descent from some earlier form, to establish the line of de-
scent is to determine in great part the classification. Much as-
sistance in this direction is derived from embryology, or the
history of the development of the individual {ontogeny) • so
that it may be said that the ontogeny indicates, though it does
not actually determine, the line of descent (phytogeny) ; and
it is owing to the importance of this truth that naturalists have
in recent years given so much attention to comparative embry-
ology.
It will be inferred that a natural system of classification must
be based both on function and structure, though chiefly on the
latter, since organs of very different origin may have a similar
function ; or, to express this otherwise, homologous structures
may not be analogous ; and homology gives the better basis for
classification. To illustrate, the wing of a bat and a bird are
both homologous and analogous ; the wing of a butterfly is
analogous but not homologous with these ; manifestly, to clas-
sify bats and birds together would be better than to put birds
and insects in the same group, thus leaving other points of re-
lationship out of consideration.
The broadest possible division of the animal kingdom is into
groups, including respectively one-celled and many-celled forms
— i. e., into Protozoa and Metazoa. As the wider the grouping
the less are differences considered, it follows that the more sub-
divided the groups the more complete is the information con-
veyed ; thus, to say that a dog is a metazoan is to convey a cer-
tain amount of information ; that he is a vertebrate, more ; that
he is a mammal, a good deal more, because each of the latter
terms includes the former.
36
COMPARATIVE PHYSIOLOGY.
Inverte-
brata.
Animal
Kingdom. |
r Protozoa (amoeba, vorticella, etc.).
Ccelenterata (sponges, jelly-fish, polyps, etc.).
I Echinodermata (star-fish, sea-urchins, etc.).
J Vermes (worms).
1 Ai'thropods (crabs, insects, spiders, etc.).
Mollusca (oysters, snails, etc.).
Molluscoidea (moss-like animals).
*- Tunicata (ascidians).
( Pisces (fishes).
Amphibia (frogs, menobranehus, etc.).
Vertebrata. \ Keptilia (snakes, turtles, etc.).
Aves (birds').
Mammalia (domestic quadrupeds, etc.).
I
The above classification (of Claus) is, like all such arrange-
ments, but the expression of one out of many methods of view-
ing the animal kingdom.
For the details of classification and for the grounds of that
we have presented, we refer the student to works on zoology ;
but we advise those who are not familiar with this subject,
when a technical term is used, to think of that animal belong-
ing to the group in question with the structure of which they
are best acquainted.
Man's Place in the Animal Kingdom.
It is no longer the custom with zoologists to place man in an
entirely separate group by himself ; but he is classed with the
primates, among which are also grouped the anthropoid apes
(gorilla, chimpanzee, orang, and the gibbon), the monkeys of
the Old and of the New World, and the lemurs. So great is
the structural resemblance of man and the other primates that
competent authorities declare that there is more difference be-
tween the structure of the most widely separated members of
the group than between certain of the anthropid apes and man.
The points of greatest resemblance between man and the
anthropoid apes are the following : The same number of verte-
brae ; the same general shape of the pelvis ; a brain distinguish-
ing them from other mammals ; and posture, being bipeds.
The distinctive characters are size, rather than form of the
brain, that of man being more than twice as large ; a relatively
larger cranial base, by which, together with the greater size of
the jaws, the face becomes prominent ; the earlier closure of
the sutures of the cranium, arresting the growth of the brain ;
more developed canine teeth and difference in the order of erup-
tion of the permanent teeth ; the more posterior position of the
foramen magnum ; the relative length of the limbs to each
GENERAL BIOLOGY. 37
other and the rest of the body ; minor differences in the hands
and feet, especially the greater freedom and power of apposition
of the great-toe.
But the greatest distinction between man and even his closest
allies among the apes is to be found in the development to an
incomparably higher degree of his intellectual and moral na-
ture, corresponding to the differences in weight and structure
of the human brain, and associated with the use of spoken and
written language ; so that the experience of previous genera-
tions is not only registered in the organism (heredity), but in
the readily available form of books, etc.
The greatest structural difference between the races of men
are referable to the cranium ; but, since they all interbreed
freely, they are to be considered varieties of one species.
THE LAW OF PERIODICITY OR RHYTHM IN NATURE.
The term rhythm to most minds suggests music, poetry, or
dancing, in all of which it forms an essential part so simple,
pronounced, and uncomplicated as to be recognized by all with
ease.
The regular division of music into bars, the recurrence of
chords of the same notes at certain intervals, of forte and piano,
seem to be demanded by the very nature of the human mind.
The same applies to poetry. Even a child that can not under-
stand the language used, or an adult listening to recitations in
an unknown tongue, enjoys the flow and recurrences of the
sounds. Dancing has in all ages met a want in human organi-
zations, which is partly supplied in quieter moods by the regu-
larity of the steps in walking and similar simple movements.
But as rhythm runs through all the movements of animals,
so is it also found in all literature and all art. Infinite variety
wearies the mind, hence the fatigue felt by the sight-seer. Re-
currence permits of repose, and gratifies an established taste or
appetite. The mind delights in what it has once enjoyed, in
repetition within limits. Repetition with variety is manifestly
a condition of the growth and development of the mind. This
seems to apply equally to the body, for every single function of
each organism, however simple or complex it may be, exempli-
fies this law of periodicity. The heart's action is rhythmical
(beats) ; the blood flows in intermitting gushes from the central
pump ; the to-and-fro movements of respiration are so regular
38 COMPARATIVE PHYSIOLOGY.
that their cessation would arouse the attention of the least in-
structed ; food is demanded at regular intervals ; the juices of
the digestive tract are poured out, not constantly but period-
ically ; the movements by which the food is urged along its
path are markedly rhythmic ; the chemical processes of the
body wax and wane like the fires in a furnace, giving rise to
regular augmentations of the temperature of the body at fixed
hours of the day, with corresponding periods of greatest bodily
activity and the reverse.
This principle finds perfect illustration in the nervous sys-
tem. The respiratory act of the higher animals is effected
through muscular movements dependent on regular waves of
excitation reaching them along the nerves from the central cells
which regularly discharge their forces along these channels.
Were not the movements of the body periodic or rhythmical,
instead of that harmony which now prevails, every muscular
act would be a convulsion, though even in the movements of
the latter there is a highly compounded rhythm, as a noise is
made up of a variety of musical notes. The senses are subject
to the same law. The eye ceases to see and the ear to hear and
the hand to feel if continuously stimulated; and doubtless in
all art this law is unconsciously recognized. That ceases to be
art which fails to provide for the alternate repose and excita-
tion of the senses. The eye will not tolerate continuously one
color, the ear the same sound. Why is a breeze on a warm day
so refreshing ? The answer is obvious.
Looking to the world of animate nature as a whole, it is
noticed that plants have their period of sprouting, flowering,
seeding, and decline; animals are born, pass through various
stages to maturity, diminish in vigor, and die. These events
make epochs in the life-history of each species ; the recurrence
of which is so constant that the agricultural and other arrange-
ments even of savages are planned accordingly. That the in-
dividuals of each animal group have a definite period of dura-
tion is another manifestation of the same law.
Superficial observation suffices to furnish facts which show
that the same law of periodicity is being constantly exemplified
in the world of inanimate things. The regular ebb and flow of
the tides ; the rise and subsidence of rivers ; the storm and the
calm; summer and winter; day and night — are all recurrent,
none constant.
Events apparently without any regularity, utterly beyond
GENERAL BIOLOGY. 39
any law of recurrence, when sufficiently studied are found to
fall under the same principle. Thus it took some time to learn
that volcanic eruptions occurred with a very fair degree of
regularity.
In judging of this and all other rhythmical events it must
be borne in mind that the time standard is for an irregularity
that seems large, as in the instance just referred to, becomes
small when considered in relation to the millions of years of
geological time; while in the case of music a trifling irregu-
larity, judged by fractions of a second, can not be tolerated by
the musical organization — which is equivalent to saying that
the interval of departure from exact regularity seems large.
As most of the rhythms of the universe are compounded of
several, it follows that they may seem, until closely studied,
very far from regular recurrences. This may be observed in
the interference in the regularity of the tides themselves, the
daily changes of which are subject to an increase and decrease
twice in each month, owing to the influence of the sun and
moon being then either coincident or antagonistic.
In the functions of plants and animals, rhythms must be-
come very greatly compounded, doubtless often beyond recog-
nition.
Among the best examples of rhythm in animals are daily
sleep and winter sleep, or hibernation ; yet, amid sleep, dreams
or recurrences of cerebral activity are common — that is, one
rhythm (of activity) overlies another (of repose). In like man-
ner many hibernating animals do not remain constantly in their
dormant condition throughout the winter months, but have
periods of wakefulness ; the active life recurs amid the life of
functional repose.
To return to the world of inanimate matter, we find that the
crust of the earth itself is made of layers or strata the result of
periods of elevation and depression, of denudation and deposi-
tion, in recurring order.
The same law is illustrated by the facts of the economic and
other conditions of the social state of civilized men. Periods
of depression alternate with periods of revival in commercial
life.
There are periods when many more marriages occur and
many more children are born, corresponding with changes in
the material conditions which influence men as well as other
animals.
40 COMPARATIVE PHYSIOLOGY.
Finally, and of special interest to the medical student, are
the laws of rhythm in disease. Certain fevers have their regu-
lar periods of attack, as intermittent fever; while all diseases
have their periods of exacerbation, however invariable the
symptoms may seem to be to the ordinary observer or even to
the patient himself.
Doubtless the fact that certain hereditary diseases do not
appear in the offspring at once, but only at the age at which
they were manifested in the parents, is owing to the same
cause.
Let us now examine more thoroughly into the real nature of
this rhythm which prevades the entire universe.
If a bow be drawn across a violin-string on which some small
pieces of paper have been placed, these will be seen to fly off ;
and if the largest string be experimented upon, it can be ob-
served to be in rapid to-and-fro motion, known as vibration,
which motion is perfectly regular, a definite number of move-
ments occurring within a measured period of time ; in other
words the motion is rhythmical. In strings of the finest size
the motion is not visible, but we judge of its existence because
of the result, which is in each instance a sound. Sound is to us,
however, an affection of the nerve of hearing and the brain,
owing to the vibrations of the ear caused by similar vibrations
of the violin-strings. The movements of the nerves and nerve-
cells are invisible and molecular, and we seem to be justified in
regarding molecular movements as constant and associated
with all the properties of matter whether living or dead.
We see, then, that all things living and lifeless are in con-
stant motion, visible or invisible ; there is no such thing in the
universe as stable equilibrium. Change, ceaseless change, is
written on all things ; and, so far as we can judge, these
changes, on the whole, tend to higher development. Neither
rhythm, however, nor anything else, is perfect. Even the mo-
tions of planets are subject to perturbations or irregularities
in their periodicity. This subject is plainly boundless in its
scope. We have introduced it at this stage to prepare for its
study in detail in dealing with each function of the animal
body. If we are correct as to the universality of the law of
rhythm, its importance in biology deserves fuller recognition
than it has yet received in works on physiology ; it will, ac-
cordingly, be frequently referred to in the future chapters of
this book.
GENERAL BIOLOGY. 41
THE LAW OF HABIT.
Every one must have observed in himself and others the
tendency to fall into set ways of doing certain things, in which
will and clear pui'pose do not come prominently into view.
Further observation shows that the lower animals exhibit this
tendency, so that, for example, the habits of the horse or the dog
may be an amusing reflection of those of the master. Trees are
seen to bend permanently in the direction toward which the
prevailing winds blow.
The violin that has experienced the vibrations, aroused by
some master's hand acquires a potential musical capability not
possessed by an instrument equally good originally, but the
molecular movements of which never received such an educa-
tion.
It appears, then, that underlying what we call habit, there is
some broad law not confined to living things ; indeed, the law
of habit appears to be closely related to the law of rhythm we
have already noticed. Certain it is that it is inseparable from
all biological phenomena, though most manifest in those organ-
isms provided with a nervous system, and in that system itself.
What we usually call habit, however expressed, has its physical
correlation in the nervous system. We may refer to it in this
connection later: but the subject has relations so numerous
and fundamental that it seems eminently proper to introduce
it at this early stage, forming as it does one of those corner-
stones of the biological building on which the superstructure
must rest.
When we seek to come to a final explanation of habit in this
case, as in most others, in which the fundamental is involved,
we are soon brought against a wall over which we ai*e unable
to climb, and through which no light comes to our intellects.
We must simply believe, as the result of observation, that it
is a law of matter, in all the forms manifested to us, to assume
accustomed modes of behavior, perhaps we may say molecular
movement, in obedience to inherent tendencies. But, to recog-
nize this, throws a flood of light on what would be inexplicable,
even in a minor degree. We can not explain gravitation in it-
self ; but, assuming its universality, replaces chaos by order in
our speculations on matter.
Turning to living matter, we look for the origin of habit in
the apparently universal principle that primary molecular
42 COMPARATIVE PHYSIOLOGY.
movement in one direction renders that movement easier after-
ward, and in proportion to the frequency of repetition ; which
is equivalent to saying that functional activity facilitates func-
tional activity. Once accepting this as of universal application
in biology, we have an explanation of the origin, the compara-
tive rigidity, and the necessity of habit. There must be a phys-
ical basis or correlative of all mental and moral habits, as well
as those that may be manifested during sleep, and so purely in-
dependent of the will and consciousness. We are brought, in
fact, to the habits of cells in considering those organs, and that
combination of structures which makes up the complex individ-
ual mammal. It is further apparent that if the cell can trans-
mit its nature as altered by its experiences at all, then habits
must be hereditary, which is known to be the case.
Instincts seem to be but crystallized habits, the inherited
results of ages of functional activity in certain well-defined
directions.
To a being with a highly developed moral nature like man,
the law of habit is one of great, even fearful significance. We
make to-day our to-morrow, and in the present we are deciding
the future of others, as our present has been made for us in part
by our ancestors. We shall not pursue the subject, which is of
boundless extent, further now, but these somewhat general
statements will be amplified and applied in future chapters.
THE ORIGIN OF THE FORMS OF LIFE.
It is a matter of common observation that animals originate
from like kinds, and plants from forms resembling themselves ;
while most carefully conducted experiments have failed to show
that living matter can under any circumstances known to us
arise from other than living matter.
That in a former condition of the universe such may have
been the case has not been disproved, and seems to be the logical
outcome of the doctrine of evolution as applied to the universe
generally.
By evolution is meant the derivation of more complex and
differentiated forms of matter from simpler and more homogene-
ous ones. When this theory is applied to organized or living
forms, it is termed organic evolution. There are two views of
the origin of life : the one, that each distinct group of plants
and animals was independently created ; while by " creation " is
GENERAL BIOLOGY. 43
simply meant that they came into being in a manner we know
not how, in obedience to the will of a First Cause. The other
view is denominated the theory of descent with modification,
the theory of transmutation, organic evolution, etc., which
teaches that all the various forms of life have been derived
from one or a few primordial forms in harmony with the recog-
nized principles of heredity and variability. The most widely
known and most favorably received exposition of this theory is
that of Charles Darwin, so that his views will be first presented
in the form of a hypothetical case. Assume that one of a group
of living forms varies from its fellows in some particular, and
mating with another that has similarly varied, leaves progeny
inheriting this characteristic of the parents, that tends to be
still further increased and rendered permanent by successive
pairing with forms possessing this valuation in shape, color, or
whatever it may be. We may suppose that the variations may
be numerous, but are always small at the beginning. Since all
animals and plants tend to multiply faster than the means of
support, a competition for the means of subsistence arises, in
which struggle the fittest, as judged by the circumstances, al-
ways is the most successful ; and if one must perish outright, it
is the less fit. If any variation arises that is unfavorable in
this contest, it will render the possessor a weaker competitor :
hence it follows that only useful variations are preserved. The
struggle for existence is, however, not alone for food, but for
anything which may be an advantage to its possessor. One form
of the contest is that which results from the rivalry of members
of the same sex for the possession of the females ; and as the
female chooses the strongest, most beautiful, most active, or the
supreme in some respect, it follows that the best leave the great-
est number of progeny. This has been termed sexual selection.
In determining what forms shall survive, the presence of
other plants or animals is quite as important as the abundance
of food and the physical conditions, often more so. To illustrate
this by an example : Certain kinds of clover are fertilized by
the visits of the bumble-bee alone ; the numbers of bees exist-
ing at any one place depends on the abundance of the field-mice
which destroy the nests of these insects ; the numbers of mice
will depend on the abundance of creatures that prey on the
mice, as hawks and owls ; these, again, on the creatures that
specially destroy them, as foxes, etc. ; and so on, the chain of
connections becoming more and more lengthy.
44
COMPARATIVE PHYSIOLOGY.
Fig. 47.— Shows the embryos of four mammals in the three eorresponding stages : of
a hog (II), calf (C), rabbit (R), and a man (M). The conditions of the three differ-
ent stages of development, which the three cross-rows (I, II, III) represent, are
selected to correspond as exactly as possible. The first, or upper cross-row, I.
represents a very early stage, with gill-openings, and without limbs. The second
(middle) cross-row, II, shows a somewhat later stage, with the first rudiments of
limbs, while the gill-openings are yet retained. The third (lowest) cross-row, III,
shows a still later stage, with the limbs more developed and the gill-openings
GENERAL BIOLOGY. 45
lost. The membranes and appendages of the embryonic body (the amnion, yelk-
sac, allantois) are omitted. The whole twelve figures are slightly magnified, the
upper ones more than the lower. To facilitate "the comparison, they are all re-
duced to nearly the same size in the cuts. All the embryos are seen from the left
side ; the head extremity is above, the tail extremity below ; the arched back
turned to the right, The letters indicate the same parts in all the twelve figures,
namely: v, fore-brain; z, twist-brain; m, mid-brain; h, hind-brain; re, after-brain;
r, spinal marrow- e, nose; «, eye; o, ear; k, gill-arches; g. heart; iv, vertebral
column; /, fore-limbs; b, hind-limbs; s, tail. (After Haeckel.)
If a certain proportion of forms varying similarly were sep-
arated by any great natural barrier, as a chain of lofty mountains
or an intervening body of water of considerable extent, and so pre-
vented from breeding with forms that did not vary, it is clear that
-there would be greater likelihood of their differences being pre-
served and augmented up to the point of their greatest usefulness.
We may now inquire whether such has actually been the
course of events in nature. The evidence may be arranged un-
der the following heads :
1. Morphology. — Briefly, there is much that is common to
entire large groups of animals ; so great, indeed, are the resem-
blances throughout the whole animal kingdom that herein is
found the strongest argument of all for the doctrine of descent.
To illustrate by a single instance — fishes, reptiles, birds, and
mammals possess in common a vertebral column bearing the
same relationship to other parts of the animal. It is because of
resemblances of this kind, as well as by their differences, that
naturalists are enabled to classify animals.
2. Embryology.— In the stages through which animals pass
in their development from the ovum to the adult, it is to be ob-
served that the closer the resemblance of the mature organism
in different groups, the more the embryos resemble one another.
Up to a certain stage of development the similarity between
groups of animals, widely separated in their post-embryonic
life, is marked : thus the embryo of a reptile, a bird, and a mam-
mal have much in common in their earlier stages. The embryo
of the mammal passes through stages which represent condi-
tions which are permanent in lower groups of animals, as for
example that of the branchial arches, which are represented by
the gills in fishes. It may be said that the developmental his-
tory of the individual (ontogeny) is a brief recapitulation of the
development of the species (phylogeny). Apart from the theory
of descent, it does not seem possible to gather the true signifi-
cance of such facts, which will become plainer after the study
of the chapters on reproduction.
3. Mimicry may be cited as an instance of useful adaptation.
46 COMPARATIVE PHYSIOLOGY.
Thus, certain beetles resemble bees and wasps, which latter are
protected by stings. It is believed that such groups of beetles
as these arose by a species of selection ; those escaping enemies
which chanced to resemble dreaded insects most, so that birds
which were accustomed to prey on beetles, yet feared bees, would
likewise avoid the mimicking forms.
4. Rudimentary Organs. — Organs which were once func-
tional in a more ancient form, but serve no use in the creatures
in which they are now found, have reached, it is thought, their
rudimentary condition through long periods of comparative
disuse, in many generations. Such are the rudimentary mus-
cles of the ears of man, or the undeveloped incisor teeth found
in the upper jaw of ruminants.
5. Geographical Distribution.— It can not be said that ani-
mals and plants are always found in . the localities where they
are best fitted to flourish. This has been well illustrated within
the lifetime of the present generation, for the animals intro-
duced into Australia have many of them so multiplied as to
displace the forms native to that country. But, if we assume
that migrations of animals and transmutations of species have
taken place, this difficulty is in great part removed.
6. Paleontology. — The rocks bear record to the former exist-
ence of a succession of related forms; and, though all the in-
termediate links that probably existed have not been found,
the apparent discrepancy can be explained by the nature of
the circumstances under which fossil forms are preserved ; and
the "imperfection of the geological record."
It is only in the sedimentary rocks arising from mud that
fossils can be preserved, and those animals alone with hard
parts are likely to leave a trace behind them ; while if these
sedimentary rocks with their inclosed fossils should, owing to
enormous pressure or heat be greatly changed (metamorphosed),
all trace of fossils must disappear — so that the earliest forms
of life, those that would most naturally, if preserved at all, be
found in the most ancient rocks, are wanting, in consequence
of the metamorphism which such formations have undergone.
Moreover, our knowledge of the animal remains in the earth's
crust is as yet very incomplete, though, the more it is explored,
the more the evidence gathers force in favor of organic evolu-
tion. But it must be remembered that those groups constitut-
ing species are in geological time intermediate links.
7. Fossil and Existing Species.— If the animals and plants
GENERAL BIOLOGY.
47
now peopling the earth were entirely different from those that
flourished in the past, the objections to the doctrine of descent
would be greatly strengthened ; but when it is found that there
is in some cases a scarcely broken succession of forms, great
force is added to the arguments by which we are led to infer
the connection of all forms with one another.
To illustrate by a single instance: the existing group of
horses, with a single toe to each foot, was preceded in geological
Fig. 48. — Bones of the feet of the different genera of Equidm (after Marsh), a, foot
of Orohippus (Eocene); b, foot of Anchitlierium (Lower Miocene); c, foot of Hip-
parion (Pliocene); d, foot of the recent genus Equus.
time in America by forms with a greater number of toes, the
latter increasing according to the antiquity of the group.
These forms occur in succeeding geological formations. It is
impossible to resist the conclusion that they are related gene-
alogically (phylogenetically).
8. Progression. — Inasmuch as any form of specialization that
would give an animal or plant an advantage in the struggle for
existence would be preserved, and as in most cases when the
competing forms are numerous such would be the case, it is
possible to understand how the organisms that have appeared
have tended, on the whole, toward a most pronounced pro-
gression in the scale of existence. This is well illustrated in
the history of civilization. Barbarous tribes give way before
civilized man with the numberless subdivisions of labor he in-
stitutes in the social organism. It enables greater numbers to
flourish, as the competition is not so keen as if activities could
be exercised in a few directions only.
9. Domesticated Animals. — Darwin studied our domestic ani-
mals long and carefully, and drew many important conclusions
4S
COMPARATIVE PHYSIOLOGY.
from his researches. He was convinced that they had all been
derived from a few wild representatives, in accordance with the
principles of natural selection. Breeders have both consciously
and unconsciously, formed races of animals from stocks which
the new groups have now supplanted; while primitive man
had tamed various species which he kept for food and to assist
in the chase, or as beasts of burden. It is impossible to believe
that all the different races of dogs have originated from dis-
tinct wild stocks, for many of them have been formed within
recent periods ; in fact, it is likely that to the jackal, wolf, and
fox, must we look for the wild progenitors of our dogs. Dar-
win concluded that, as man had only utilized the materials
Nature provided in forming his races of domestic animals, he
had availed himself of the variations that arose spontaneously,
and increased and fixed them by breeding those possessing the
same variation together, so the like had occurred without his
aid in nature among wild forms.
Evolutionists are divided as to the origin of man himself ;
some, like Wallace, who are in accord with Darwin as to the
4- 3
Fia. 40.— Skeleton of hand or fore-foot of six mammals. I. man; II, dog; III, pit;;
IV, ox; V, tapir; VI, horse, r, radius; u, ulna; a, scaphoid; b, semi-lunur; c,
Iriquetrum (cuneiform); d, trapezium; e, trapezoid; /, capitatum (unciform pro-
cess); /•/, hamatum (unciform bone); p, pisiform; ij thumb; 2, digit; 3, middle
finger; 4, ring-finger; 5, little finger. (After Gegenbaur.)
origin of living forms in general, believe that the theory of
natural selection does not suffice to account for the intellectual
GENERAL BIOLOGY.
49
50 COMPARATIVE PHYSIOLOGY.
and moral nature of man. Wallace believes that man's body
has been derived from lower forms, but that his higher nature
is the result of some unknown law of accelerated development ;
while Darwin, and those of his way of thinking, consider that
mau in his entire nature is but a grand development of powers
existing in minor degree in the animals below him in the scale.
Summary. — Every group of animals and plants tends to in-
crease in numbers in a geometrical progression, and must, if
unchecked, overrun the earth. Every variety of animals and
plants imparts to its offspring a general resemblance to itself,
but with minute variations from the original. The variations
of offsprings may be in any direction, and by accumulation
constitute fixed differences by which a new group is marked
off. In the determination of the variations that persist, the law
of survival of the fittest operates.
REPRODUCTION.
As has been already noticed, protoplasm, in whatever form,
after passing through certain stages in development, undergoes
a decline, and finally dies and joins the world of unorganized
matter ; so that the permanence of living things demands the
constant formation of new individuals. Groups of animals
and plants from time to time become extinct ; but the lifetime
of the species is always long compared with that of the indi-
vidual. Reproduction by division seems to arise from an exi-
gency of a nutritive kind, best exemplified in the simpler or-
ganisms. When the total mass becomes too great to be supported
by absorption of pabulum from without by the surface of the
body, division of the organism must take place, or death ensues.
It appears to be a matter of indifference how this is accom-
plished, whether by fission, endogenous division, or gemmation,
so long as separate portions of protoplasm result, capable of
leading an independent existence. The very undifferentiated
character of these simple forms prepares us to understand how
each fragment may go through the same cycle of changes as
the parent form. In such cases, speaking generally, a million
individuals tell the same biological story as one ; yet these
must exist as individuals, if at all, and not in one great united
mass. But in the case of conjugation, which takes place some-
times in the same groups as also multiply by division in its
various forms, there is plainly an entirely new aspect of the
case presented. We have already shown that no two cells, how-
ever much alike they may seem as regards form and the cir-
cumstances under which they exist, can have, in the nature of
the case, precisely the same history, or be the subjects of ex-
actly the same experiences. We have also pointed out that all
these phenomena of cell-life are known to us only as adapta-
tions of internal to external conditions ; for, though we may not
be always able to trace this connection, the inference is justi-
52 COMPARATIVE PHYSIOLOGY.
Sable, because there are no facts known to xis that contradict
such an assumption, while those that are within our knowledge
bear out the generalization. We have already learned that liv-
ing things are in a state of constant change, as indeed are all
things ; we have observed a constant relation between certain
changes in the environment, or sum total of the surrounding
conditions, as, for example, temperature, and the behavior of
the protoplasm of plants and animals ; so that we must believe
that any one form of protoplasm, however like another it may
seem to our comparatively imperfect observation, is different
in some respects from every other — as different, relatively, as
two human beings living in the same community during the
whole of their lives ; and in many cases as unlike as individuals
of very different nationality and history. "We are aware that
when two such persons meet, provided the unlikeness is not so
great as to prevent social intercourse, intercommunication may
prove very instructive. Indeed, the latter grows out of the
former; our illustration is itself explained by the law we are
endeavoring to make plain. It would appear, then, that con-
tinuous division of protoplasm without external aid is not pos-
sible ; but that the vigor necessary for this must in some way
be imparted by a particle (cell) of similar, yet not wholly like,
protoplasm. This seems to furnish an explanation of the neces-
sity for the conjugation of living forms, and the differentiation
of sex. Very frequently conjugation in the lowest animals and
plants is followed by long periods when division is the prevail-
ing method of reproduction. It is worthy of note, too, that
when living forms conjugate, they both become quiescent for a
longer or shorter time. It is as though a period of preparation
preceded one of extraordinary activity. We can at present
trace only a few of the steps in this rejuvenation of life-stuff.
Some of these have been already indicated, which, with others,
will now be further studied in this division of our subject, both
because reproduction throws so much light on cell-life, and be-
cause it is so important for the understanding of the physio-
logical behavior of tissues and organs. It may be said to be
quite as important that the ancestral history of the cells of an
organism be known as the history of the units composing a
community. A, B, and C, can be much better understood if
we know something alike of the history of their race, their an-
cestors, and their own past; so is it with the study of any indi-
vidual animal, or group of animals or plants. Accordingly,
REPRODUCTION. 53
embryology, or the history of the origin and development of
tissues and organs, will occupy a prominent place in the vari-
ous chapters of this work. The student will, therefore, at the
outset be furnished with a general account of the subject, while
many details and applications of principles will be left for the
chapters that treat of the functions of the various organs of
animals. The more knowledge the student possesses of zoology
the better, while this science will appear in a new light under
the study of embryology.
Animals are divisible, according to general structure, into
Protozoa, or unicellular animals, and Metazoa, or multicellular
forms — that is, animals composed of cell aggregates, tissues, or
organs. Among the latter one form of reproduction appears
for the first time in the animal kingdom, and becomes all but
universal, though it is not the exclusive method ; for, as seen in
Hydra, both this form of generation and the more primitive
gemmation occur. It is known as sexual multiplication, which
usually, though not invariably, involves conjugation of two un-
like cells which may arise in the same or different individuals.
That these cells, known as the male and female elements, the
ovum and the spermatozoon, are not necessarily radically dif-
ferent, is clear from the fact that they may arise in the one in-
dividual from the same tissue and be mingled together. These
cells, however, like all others, tell a story of continual progress-
ive differentiation corresponding to the advancing evolution of
higher from lower forms. Thus hermaphroditism, or the coex-
istence of organs for the production of male and of female cells
in the same individual, is confined to invertebrates, among
which it is rather the exception than the rule. Moreover, in
such hermaphrodite forms the union of cells with greater differ-
ence in experiences is provided for by the union of different in-
dividuals, so that commonly the male cell of one individual
unites with (fertilizes) the female cell of a different individual.
It sometimes happens that among the invertebrates the cells
produced in the female organs of generation possess the power
of division, and continued development wholly independently
of the access of any male cell {parthenogenesis) ; such, how-
ever, is almost never the exclusive method of increase for any
group of animals, and is to be regarded as a retention of a more
ancient method, or perhaps rather a reversion to a past biologi-
cal condition. No instance of complete parthenogenesis is
known among vertebrates, although in birds partial develop-
54 COMPARATIVE PHYSIOLOGY.
merit of the egg may take place independently of the influence
of the male sex. The best examples of parthenogenesis are to
be found among insects and crustaceans.
It is to be remembered that, while the cells which form tbe
tissues of the body of an animal have become specialized to
discharge one particular function, they have not wholly lost
all others ; they do not remain characteristic amceboids, as we
may term cells closely resembling Amoeba in behavior, nor do
they wholly forsake their ancestral habits. They all retain the
power of reproduction by division, especially when young and
most vigorous ; for tissues grow chiefly by the production of
new cells rather than the enlargement of already mature ones.
Cells wear out and must be replaced, which is effected by the
processes already described for Amoeba and similar forms.
Moreover, there is retained in the blood of animals an army of
cells, true amceboids, ever ready to hasten to repair tissues lost
by injury. These are true remnants of an embryonic condition ;
for at one period all the cells of the organism were of this un-
differentiated, plastic character. But the cell {ovum) from
which the individual in its entirety and with all its complexity
arises mostly by the union with another cell (spermatozoon),
must be considered as one that has remained unspecialized
and retained, and perhaps increased its reproductive functions.
They certainly have become more complex. The germ-cell
may be considered unspecialized as regards other functions, but
highly specialized in the one direction of exceedingly great ca-
pacity for growth and complex division, if we take into account
the whole chain of results ; though in considering this it must
be borne in mind that after a certain stage of division each
individual cell repeats its ancestral history again ; that is to
say, it divides and gives rise to cells which progress in turn as
well as multiply. From another point of view the ovum is a
marvelous storehouse of energy, latent or potential, of course,
but under proper conditions liberated in varied and unexpected
forms of force. It is a sort of reservoir of biological energy
in the most concentrated form, the liberation of which in sim-
pler forms gives rise to that complicated chain of events which
is termed by the biologist development, but which may be ex-
pressed by the physiologist as the transformation of potential
into kinetic energy, or the energy of motion. Viewed chemi-
cally, it is the oft-repeated story of the production of forms, of
greater stability and simplicity, from more unstable and com-
REPRODUCTION.
55
plex ones, involving throughout the process of oxidation ; for
it niust ever be kept in mind that life and oxidation are con-
comitant and inseparable. The further study of reproduction
in the concrete will render the meaning and force of many of
the above statements clearer.
THE OVUM,
The typical female cell, or ovum, consists of a mass of pro-
toplasm, usually globular in form, containing a nucleus and
nucleolus.
The ovum may or may not be invested by a membrane ; the
protoplasm of the body of the cell is usually highly granular,
and may have stored up within it a varying amount of proteid
material (food-yelk), which has led to division of ova into
classes, according to the manner of distribution of this nutri-
tive reserve. It is either concentrated at one pole (telolecith-
al) ; toward the center (centrolecithal) ; or evenly distributed
throughout (alecithal).
During development this "f7
material is converted by x^
the agency of the cells of
the young organism (em-
bryo) into active proto-
plasm ; in a word, they
feed upon and assimilate
or build up this food-stuff
into their own substance,
as Amoeba does with any
proteid material it appro-
priates.
The nucleus (germinal
vesicle) is large and well
defined, and contains with-
in itself a highly refractive
nucleolus (germinal spot).
These closely resemble in general the rest of the cell, but stain
more deeply and are chemically different in that they contain
nucleine (nucleoplasm, chromatin >.
It will be observed that the ovum differs in no essential par-
ticular of structure from other cells. Its differences are hidden
ones of molecular structure and functional behavior. In ac-
Fig. 55. — Semi-diagrammatic representation of
a mammalian ovum (Sch&fer). Highly mag-
nified. z}>, zonapellucida; in, vitellus (yelk);
gv, germinal vesicle; gs, germinal spot.
56
COMPARATIVE PH YSIOLOG Y,
cordance with the diverse circumstances under which ova ma-
ture and develop, certain variations in structure, mostly of the
nature of additions, present themselves.
Thus, ova may be naked, or provided with one or more cover-
ings. In vertebrates there are usually two membranes around
the protoplasm of the ovum : a delicate covering (Vitelline
membrane) beneath which there is another, which is sieve-like
from numerous perforations (zona radiata, or z. pellucida).
The egg membrane may be impregnated with lime salts (shell).
Between the membranes and the yelk there is a fluid albumi-
nous substance secreted by the glands of the oviduct, or by other
special glands, which provide proteid nutriment in different
physical condition from that of the yelk.
The general naked-eye appearances of the ovum may be
learned from the examination of a hen's egg, which is one of
11
wy-
yy
eh. I
Fig. 56. — Diagrammatic section of an unimpregnated fowl's egg (Foster and Balfour,
after Allen Thomson), bl, blastoderm or cicatricnla; 10. y, white yelk; y. y, yel-
low yelk; ch.l. chalaza; i.s.m, inner layer of shell membrane; s. m, outer layer
of shell membrane; s. shell; a. c. h, air-space: w, the white of the egg; v. (, vitel-
line membrane ; x, the denser albuminous layer lying next the vitelline mem-
brane.
the most complicated known, inasmuch as it is adapted for
development outside of the body of the mother, and must, con-
sequently, be capable of preserving its form and essential vital
properties in a medium in which it is liable to undergo loss of
water, protected as it now is with shell, etc., but which, at the
REPRODUCTION. 57
same time permits the entrance of oxygen and moisture, and
conducts heat, all being- essential for the development of the
germ within this large food-mass. The shell serves, evidently,
chiefly for protection, since the eggs of serpents (snakes, turtles,
etc.) are provided only with a very tough membranous cover-
ing, this answering every purpose in eggs buried in sand or
otherwise protected as theirs usually are. As the hen's egg is
that most readily studied and most familiar, it may be well to
describe it in somewhat further detail, as illustrated in the
above figure, from the examination of which it will be ap-
parent that the yelk itself is made up of a white and yellow
portion distributed in alternating zones, and composed of cells
of different microscopical appearances. The clear albumen is
structureless.
The relative distribution, and the nature of the accessory or
non-essential parts of the hen's egg, will be understood when it
is remembered that, after leaving its seat of origin, which will
be presently described, the ovum passes along a tube (oviduct)
by a movement imparted to it by the muscular walls of the
latter, similar to that of the gullet during the swallowing of
food ; that this tube is provided with glands which secrete in
turn the albumen, the membrane (outer), the lime salts of the
shell, etc. The twisted appearance of the rope-like structures
(chalazce) at each end is owing to the spiral rotatory movement
the egg has undergone in its descent.
The air-chamber at the larger end is not present from the
first, but results from evaporation of the fluids of the albumen
and the entrance of atmospheric air after the egg has been laid
some time.
THE ORIGIN AND DEVELOPMENT OF THE OVUM.
Between that protrusion of cells which gives rise to the bud
which develops directly into the new individual, and that which
forms the ovary within which the ovum as a modified cell arises,
there is not in Hydra much difference at first to be observed.
In the mammal, however, the ovary is a more complex struct-
ure, though, relatively to many organs, still simple. It consists,
n the main, of connective tissue supplied with vessels and nerves
inclosing modifications of that tissue (Graafian follicles) within
which the ovum is matured. The ovum and the follicles arise
from an inversion of epithelial cells, on a portion of the body
58
COMPARATIVE PHYSIOLOGY.
cavity (germinal ridge), which give rise to the oviim itself, and
the other cells surrounding it in the Graafian follicle. At first
these inversions form
tubules (egg-tubes) which
latter become broken up
into isolated nests of
cells, the forerunners of
the Graafian follicles.
The Graafian follicle
consists externally of a
fibrous capsule (tunica
fibrosa), in close relation
to which is a layer of cap-
illary blood-vessels (tu-
nica vasculosa), the two
together forming the gen-
eral covering (tunica
propria) for the more
delicate and important
cells within. Lining the
Fig. 57. — Section through portion of the ovary of , . . n „ ..
mammal, illustrating mode of development of tunic IS a layer Of small,
the Graafian follicles (Wiedersheim). D, dis- "„__^,__-i,„+ „,,-k:„„i „„iio,
cue proligerus ; El, ripe ovum; G, follicular SOmewnat CUDlcai cells
cells of germinal epithelium; g, blood-vessels; /anovnhvrivtri rtvnv><)i7n<zri}
K, germinal vesicle (nucleus) and germinal Wiemorana granulosa),
spot (nucleolus) ; KE, germinal epithelium; which at One part invest
Lf, liquor foil iculi; Mg, membrana or tunica
granulosa, or follicular epithelium; Mp, zona the OVUm Several layers
pellucida ; PS, ingrowths from the germinal <. , 7. T • x
of which deep (discus proligerus),
epithelium, ovarian tubes, by means
some of the nests retain their connection with
the epithelium; S. cavity which appears with-
in the Graafian follicle; So, stroma of ovary;
Tf, theca folliculi or capsule ; U, primitive
ova. When an ovum with its surrounding
cells has become separated from a nest, it is
known as a Graafian follicle.
while the remainder of
the space is filled by a
fluid (liquor folliculi)
probably either secreted
by the cells themselves,
or resulting from the disintegration of some of them, or both.
In viewing a section of the ovary taken from a mammal at
the breeding-season, ova and Graafian follicles may be seen in
all stages of development — those, as a rule, nearest the surface
being the least matured. The Graafian follicle appears to pass
inward, to undergo growth and development and again retire
toward the exterior, where it bursts, freeing the ovum, which is
conducted to the site of its future development by appropriate
mechanism to be described hereafter.
Changes in the Ovum itself.— The series of transformations
that take place in the ovum before and immediately after the
REPRODUCTION.
59
access of the male element is, in the opinion of many biologists,
of the highest significance, as indicating the course evolution
m
'i-r-'-^ZX? Sill
Mw/M
Fig. 58.— Sagittal section of the ovary of an adult bitch (after Waldeyerl. o. e, ova-
rian epithelium; o. t, ovarian tubes; y.f, younger follicles; o.f, older follicle;
d. v, discus proligerus, with the ovum; e, epithelium of a second ovum in the same
follicle: f. c, fibrous coat of the follicle; p. c, proper coat of the follicle; e. f. epi-
thelium.of the follicle (membrana granulosa); a./, collapsed atrophied follicle;
b. /'.blood-vessels; c. t, cell-tubes of the parovarium divided longitudinally and
transversely; /.(/.tubular depression of the ovarian epithelium in the tissue of
the ovary; b. e, beginning of the ovarian epithelium, close to the lower border of
the ovary.
has followed in the animal kingdom, as well as instructive in
illustrating the behavior of nuclei generally.
60
COMPARATIVE PHYSIOLOGY.
The germinal vesicle may acquire powers of slow movement
(amoeboid), and the germinal spot disappear : the former passes
to one surface (pole) of the ovum ; both these structures may
undergo that peculiar form of rearrangement (karyokinesis)
which may occur in the nuclei and nucleoli of other cells prior
to division ; in other words, the ovum has features common to it
and many other cells in that early stage which precedes the com-
plicated transformations which constitute the future history of
the ovum.
A portion of the changed nucleus (aster) with some of the
protoplasm of the cell accumulates at one surface (pole), which
is termed the upper pole because it is at this region that the epithe-
lial cells will be iiltimately developed, and is separated. This pro-
cess is repeated. These bodies (polar cells, polar globules, etc.),
Fig. 59.— Formation of polar cells in a star-fish (Asterias glacialis) (from Geddes,
A — K after Fol, L after O. Hertwig). A, ripe ovum with eccentric germinal vesi-
cle and spot; B — D. gradual metamorphosis of germinal vesicle and spot, as seen
in the living egg, into two asters; F, formation of first polar cells and withdrawal
of remaining part of nuclear spindle within the ovum; G, surface view of living
ovum in the first polar cell; H, completion of second polar cell; I, a later stage,
showing the remaining internal half of the spindle in the form of two clear vesi-
cles; K, ovum with two polar cells and radial stria? round female pronucleus, as
seen in the living egg (E. F, H, and I from picric acid preparations); L, expulsion
of the first polar cell. (Haddon.)
then, are simply expelled ; they take no part in the development
of the ovum ; and their extrusion is to be regarded as a prepar-
ation for the progress of the cell, whether this event follows or
precedes the entrance of the male cell into the ovum. It is wor-
thy of note that the ovum may become amceboid in the region
from which the polar globules are expelled.
The remainder of the nucleus( female pronucleus) now passes
inward to undergo further changes of undoubted importance,
possibly those by virtue of which all the subsequent evolution
of the ovum is determined. This brings us to the consideration
of another cell destined to play a brief but important role on the
biological stage.
REPRODUCTION.
61
THE MALE CELL (SPERMATOZOON).
This cell, almost without exception, consists of a nucleus
(head) and vibratile cilium. However, as indicating that the
Fig. 60.— Spermatozoa (after Haddon). Not drawn to scale. 1, sponge; 2. hydroid;
3, nematode: 4, cray-fish; 5, snail; 6, electric ray; 7, salamander; S. horse; 9, man.
In many spermatozoa, as in Nos. 7 and 9, an extremely delicate vibratile band is
present.
latter is not essential, spermatozoa without such an appendage
do occur. The obvious purpose of the cilium is to convey the
male cell to the ovum through a fluid medium — either the water
in which the ova are discharged in the case of most invertebrates,
or through the fluids that overspread the surfaces of the female
generative organs.
The Origin of the Spermatozoon.— The structures devoted to
the production of male cells (testes), when reduced to their es-
sentials, consist of tubules, of great length in mammals, lined
62
COMPARATIVE PHYSIOLOGY.
with nucleated epithelial cells, from which, by a series of
changes figured above, a general idea of their development may
be obtained.
It will be observed that throughout the series the nucleus of
the cell is in every case preserved, and finally becomes the head
fc'iG. 61.— Spermatogenesis. A— H, isolated sperm-celis of the rat, showing the devel-
opment of the spermatozoon and the gradual transformation of the nucleus into
the spermatozoon head. In G the seminal granule is being cast off tatter hi. U.
Brown) I— M, sperm-cells of an Elasmobranch. The nucleus ot each cell divides
into a large number of daughter-nuclei, each one of which is converted into the
rod-like head of a spermatozoon. N, transverse section of a ripe cell, showing
the bundle of spermatozoa and the passive nucleus (I— N, after Semper). O— b,
spermatogenesis in the earth-worm; O, young sperm-cell; P, the same divided
into four; (), spermatosphere with the central sperm-blastophore; R, a later stage;
S, nearly mature spermatozoa. (After Blomfleld.)
REPRODUCTION.
63
of the male cell. Once more we are led to see the importance
of this structure in the life of the cell.
Fertilization of the Ovum. — The spermatozoon, lashing its
way along, when it meets the ovum, enters it either through a
special minute gateway (micropyle), or, if this be not present—
as it is not in the ova of all animals — actually penetrates the
membranes and substance of the female cell, and continues act
ive till the female pronucleus is reached, when the head enters
and the tail is absorbed or blends with the female cell. The nu-
cleus of the male cell prior to union with the nucleus of the
F.PNt
F.PKt
-M.PN.
Fig. 6:2.— Fertilization of ovum of a mollusk (Elysia viridis). A. Ovum sending up a
protuberance to meet the spermatozoon. B. Approach of male pronucleus to
meet the female pronucleus. F. FN, female pronucleus; M. FN, male pronucleus.
S. spermatozoon.
ovum undergoes changes similar to those that the nucleus of the
ovum underwent, and thus becomes fitted for its special func-
tions as a fertilizer ; or perhaps it would be more correct to say
that these altered masses of nuclear substance mutually fertil-
ize each other, or initiate changes the one in the other which
conjointly result in the subsequent stages of the development
of the ovum. The altered male nucleus {male pronucleus), on
reaching the female pronucleus, finds it somewhat amaeboid,
a condition which may be shared in some degree by the entire
ovum. The resulting union gives rise to the new nucleus {seg-
mentation nucleus), which is to control the future destinies of
the cell ; while the cell itself, the fertilized ovum {oosperm), en-
ters upon new and marvelous changes.
In reality this process was foreshadowed in the dim past of
the history of living things by the conjugation of infusoria
and kindred animal and vegetable forms. When lower forms
(unicellular) conjugate they become somewhat amoeboid sooner
or later, and division of cell contents results. In some cases
Cseptic monads) the resulting cell may burst and give rise to a
64 COMPARATIVE PHYSIOLOGY.
shower of animal dust visible only by the highest powers of the
microscope, each particle of which proves to be the nucleus
from which a future individual arises.
The study of reproduction thus establishes the conception of
a unity of method throughout the animal and, it may be added,
the vegetable kingdom, for reproduction in plants is in all main
points parallel to that process in animals.
But why that costly loss of protoplasm by polar globules ?
For the present we shall only say that it appears necessary to
prevent parthenogenesis ; or at least to balance the share which
the male and female elements take in the work of producing a
new creature. It is to be remembered that both the male and
female lose much in the process — blood, nervous energy, etc., in
the case of the female, while the male furnishes a thousand-fold
more cells than are used. But the period when organisms are
best fitted for reproduction is that during which they are also
most vigorous, and can best afford the drain on their super-
fluous energies.
SEGMENTATION AND SUBSEQUENT CHANGES.
After the changes described in the last chapter a new epoch
in the biological history of the ovum — now the oosperm (or fer-
tilized egg) — begins. A very distinct nucleus (segmentation
nucleus) again appears, and the cell assumes a circular outline.
The segmentation or division of the ovum into usually fairly
equal parts now commences. This process can be best watched
in the microscopic transparent ova of aquatic animals which
undergo perfect development up to a certain advanced stage
in the ordinary water of the ocean, river, lake, etc., in which
the adult lives.
Segmentation among invertebrates will be first studied, and
for this purpose an ovum in which the changes are of a direct
and uncomplicated nature will be chosen.
The following figures and descriptions apply to a mollusk
(Elysia viridis). We distinguish in ova resting stages and
stages of activity. It is not, however, to be supposed that abso-
lute rest ever characterizes any living form, or that nothing is
transpiring because all seems quiet in these little biological
worlds ; for we have already seen reason for believing that life
and incessant molecular activity are inseparable. It may be
that, in the case of resting ova, changes of a more active char-
REPRODUCTION.
65
acter than usual are going on in their molecular constitution ;
hut, on the other hand, there may be really a diminution of
^I£j3____-£^?l
Fig. 63.— Primitive eggs of various animals, performing amoeboid movements (.very
much enlarged). "All primitive eggs are naked cells, capable of change of form.
Within the dark, finely granulated protoplasm ^egg-yelk) lies a large vesicular
kernel (the germ-vesicle'), and in the latter is a nucleolus (germ-spot); in the nu-
cleolus a germ-point (nucleolus) is often visible. Fig. .-1 1 — ,-1 4. The primitive
egg of a chalk sponge {Leuculmis .echinus), in four consecutive conditions of mo-
tion. Fia;. B 1 — B 8. The primitive egg of a hermit-crab (Ghondraeanthus cornu-
tux), in eight consecutive conditions of motion (after E. Van Beneden). Fig. C'l
— (75. Primitive egg of a cat in four different conditions of motion (after Pfliiger).
Fig. D. Primitive egg of a trout. Fig. E. Primitive egg of a hen. Fig. F. Primi-
tive human egg. (Haeckel.)
these activities in correspondence with the law of rhythm. This
seems the more probable. The meaning, however, of a " resting
66
COMPARATIVE PHYSIOLOGY.
stage " is the obvious one of apparent quiescence — cessation of
all kinds of movement. Then ensues rapidly and in succession
the following series of transformations : The nucleolus divides,
later the nucleus, into two parts. These new nuclei then wan-
der away from each other in opposite directions, and, losing
their character as nuclei and nucleoli, are replaced by asters
(polar stars), which seem to arise in the protoplasm of the body
Fig. 64.— Early stages of segmentation of a mollusk, Elysia viridis (drawn from the
living egg). A, oosperm in state of rest after the extrusion of the polar cells; B,
the nucleolus alone has divided; C, the nucleus is dividing; D, the nucleus, as
such, has disappeared, first segmentation furrow appears; E, later stage; F,
ofisperm divided into two distinct segmentation spheres, the clear nuclear space
in the center of the aster of granules is growing larger; G, resting stage of ap-
pressed two spheres; H, I, similar stages in the production of four spheres; K, .
formation of eight-celled stage. (Haddon.)
of the cell, and which are in close juxtaposition at first, but later
separate, the oosperm becoming amoeboid in one region at least.
A groove, which gradually deepens, appears on the surface, and
finally divides the cell into two halves, which at once become
flattened against each other. The nucleus may again be recog-
nized in the center of each polar star, while a new nucleolus
also reappears within the nucleus, when again a brief period of
rest ensues. In the division and reformation of the nucleus,
when most complicated (karyoMnesis), the changes may be gen-
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REPRODUCTION.
67
eralizecl as consisting of division and segregation, followed by
aggregation.
The subdivision (segmentation) of the cell, after the quies-
cence referred to, again commences, but in a plane at right
angles to the first, from which four spheres result, again to be
followed by the resting stage. The process continues in the
same way, so that there is a progressive increase in the number
Fig. 05.— The cleavage of a frog's egg (10 times enlarged). A, the parent-cell; B, the
first two cleavage-cells; 0.4 cells; D, 8 cells (4 animal and 4 vegetative); E. 12
cells (8 animal and 4 vegetative); F, 16 cells (8 animal and 8 vegetative); G, 24
cells (1(5 animal and 8 vegetative); H, 32 cells: /. 48 cells; A". 04 cell^: L. 96 eleav-
age-cclls; .]/, mo cleavage-cells (128 animal and 32 vegetative). (Haeckel.)
of segments, at least up to the point when a large number has
been formed. This is rather to be considered as a type of one
68 COMPARATIVE PHYSIOLOGY.
form of segmentation than as applicable to all, for even at
this early stage differences are to be noted in the mode of seg-
mentation wbich characterize effectually certain groups of ani-
mals ; but in all there is segmentation, and that segmentation
is rhythmical.
Segmentation results in the formation of a multicellular
aggregation which, sooner or later, incloses a central cavity
(segmentation cavity, blastocele). Usually this cell aggrega-
tion (blast ula, blastophere) is reduced to a single layer of invest-
ing cells.
The Gastrula.— Ensuing on the changes just described are
C
Fig. 66.— .Blastula and gastrula of amphioxus (Clans, after Hatschek). A, blastula
with flattened lower pole of larger cells; B, commencing invagination; C, gastru-
lation completed; the blastopore is still widely open, and one of the two hinder-
pole mesoderm cells is seen at its ventral lip. The cilia of the epiblast cells are
not represented.
others, which result in the formation of the gastrula, a form of
cell aggregation of great interest from its resemblance to the
Hydra and similar forms, which constitute in themselves inde-
pendent animals that never pass beyond that stage. The blas-
tula becomes flattened at one pole, then depressed, the cells at
this region becoming more columnar (histological differentia-
tion). This depression (invagination) deepens until a cavity is
formed (as when a hollow rubber ball is thrust in at one part
till it meets the opposite wall), in consequence of which a two-
layered embryo results, in which we recognize the primitive
mouth (blastopore) and digestive cavity (archenterori), the outer
layer (ectoderm) being usually separated from the inner (endo-
derm) by the almost obliterated segmentation cavity. Such a
form may be provided with cilia, be very actively locomotive,
and bear, consequently, the greatest resemblance to the perma-
nent forms of some aquatic animals.
The changes by which the segmented oosperm becomes a
gastrula are not always so direct and simple as in the above-
REPRODUCTION.
69
described case, but the'
behavior of the cells of
the blastosphere may be
hampered by a burden
of relatively foreign
matter, in the form of
food-yelk, in certain in-
stances ; so much so is
this the case that dis-
tinct modes of gastrula
formation may be rec-
ognized as dependent on
the quantity and ar-
rangement of food-yelk.
These we shall pass by
as being somewhat too
complicated for our pur-
pose, and we return to
the egg of the bird.
The Hen's Egg.— By
far the larger part of
the hen's egg is made
up of yelk ; but just
beneath the vitelline
membrane a small, cir-
cular, whitish body,
about four millimetres
in diameter, which al-
ways floats uppermost
in every portion of the
egg, may be seen. This
disk (blastoderm, cica-
tricula) in the fertilized
egg presents an outer
white rim (area ojxica),
within which is a trans-
parent zone (area jielhi-
cida), and most centrally
a somewhat elongated
structure, which marks
off the future being
itself (embryo). All
Fig. 67.— Female generative organs of the fowl
(after Dalton). A, ovary; B, Graafian follicle,
from which the egg has just been discharged;
C, yelk, entering upon extremity of oviduct:
D, E, second portion of oviduct, "in which the
chalaziferous membrane, chalazse. and albumen
are formed; .F, third portion, in which the fibrous
shell membranes are produced; Q, fourth por-
tion laid open, showing the egg completely
formed with its calcareous shell; //, canal
through which the egg is expelled.
70
COMPARATIVE PHYSIOLOGY,
these parts together constitute that portion (blastoderm) of the
fowl's egg which is alone directly concerned in reproduction,
all the rest serving for nutrition and protection. The appear-
ance of relative opacity in some of the parts marked off as ahove
is to be explained by thickening in the cell-layers of which they
are composed.
The Origin of the Fowl's Egg.— The ovary of a young but
mature hen consists of a mass of connective tissue (stroma),
Fig. 08. — Various stages in the segmentation of a fowl's
Kolliker).
abundantly supplied with blood-vessels, from which hang the
capsules which contain the ova in all stages of development, so
that the whole suggests, but for the color, a bunch of grapes in
REPRODUCTION. 71
an early stage. The ovum at first, in this case as in all others,
a single cell, becomes complex by addition of other cells (dis-
cus proligerus, etc.), which go to make up the yelk. All the
other parts of the hen's egg are additions made to it, as ex-
plained before, in its passage down the oviduct. The original
ovum remains as the blastoderm, the segmentation of which
may now be described briefly, its character being obvious from
an examination of Fig. 68, which represents a surface view of
the segmenting fertilized ovum (oosperm).
A segmentation cavity appears early, and is bounded above
by a single layer of epiblast cells and below by a single layer of
primitive hypoblast cells, which latter is soon composed of sev-
eral layers, while the segmentation cavity disappears.
The blastoderm of an unincubated but fertilized egg consists
of a layer of epiblastic cells, and beneath this a mass of rounded
cells, arranged irregularly arid lying loosely in the yelk, consti-
tuting the primitive hypoblast. After incubation for a couple
of hours, these cells become differentiated into a lower layer of
flattened cells (hypoblast), with mesoblastic cells scattered be-
Fig. 69.— Portion of section through an unincubated fowl's oOsperm (after Klein).
«, epiblast composed of a single layer of columnar cells; b, irregularly disposed
lower layer cells of the primitive hypoblast; c, larger formative cells resting on
white yelk; f, archenteron. The segmentation cavity lies between a and b, and
is nearly obliterated.
tween the epiblast and hypoblast, It is noteworthy that, in the
bird, segmentation will proceed up to a certain stage independ-
ently of the advent of the male cell, apparently indicating a
tendency to parthenogenesis.
The fowl's ovum then belongs to the class, a portion of which
alone segments and develops into the embryo (meroblastic), in
contradistinction to what happens in the mammalian ovum, the
whole of which undergoes division (holoblastic) ; a distinction
which is, however, superficial rather than fundamental, for in
reality in the fowl's egg the whole of the original pvum does
segment. This holoblastic character of the mammalian ovum
72
COMPARATIVE PHYSIOLOGY.
and its resemblance to the segmentation of those invertebrate
forms previously described may become apparent from an ex-
amination of the accompanying figures.
Fig. 70.— Sections of ovum of a rabbit, illustrating formation of the blastodermic vesi-
cle (after E. Van Beneden). A, B, C, D, are ova in successive stages of develop-
ment, zp, zonapellucida; eel, ectomeres, or outer cells; ent, entomeres, or inner
cells.
We shall return to the development of the mammalian ovum
later ; in the mean time we present the main features of develop-
ment in the bird.
Remembering that the development of the embryo proper
takes place within the pellucid area only, we point out that the
area opaca gradually extends over the entire ovum, inclosing the
yelk, so that the original disk which lay like a watch-glass on
the rest of the ovum, has grown into a sphere. That portion
of this area nearest the pellucid zone {area vasculosa) develops
blood-vessels that derive the food-supplies, which replenish the
blood as it is exhausted, from the hypoblast of the area opaca.
REPRODUCTION.
73
The first indications of future structural outlines in the em-
bryo is the formation of the primitive streak, an opaque band
Pig. 71.— Diagrammatic transverse sections through a hypothetical mammal oOsperm
(Haddon). A. The yelk of the primitive mammalian oOspenn is now lost. B.
Later stage; the non-embryonic epiblast has grown over the embryonic area to
form the covering cells; ep. epiblast of embryo; ep'. epiblast of yelk-sac; 7uj,
primitive hypoblast; y. s, yelk-sac, or blastodermic vesicle.
in the long diameter of the pellucid area, opaque in consequence
of cell accumulation in that region. Very soon a groove {primi-
tive groove) extends through-
out this band, which gradu-
ally occupies a more central
position. The relative thick-
ness of the several parts and
the arrangement of cells may
be gathered from Fig 72.
These structures are only
temporary, and those that re-
place them will be described
subsequently.
We have thus far spoken
of cells as being arranged in-
to epiblast, hypoblast, and
mesoblast. The origin of the
first two has been sufficiently
indicated. The mesoblast
forms the intermediate ger-
minal layer, and is derived
from the primitive hypoblast,
which differentiates into a
stratum of flattened cells,
situated below the others,
and constituting the later
i
Fig. T2.-Surface view of pellucid area of
blastoderm of eighteen hours (Foster and
Balfour). //. medullary folds ; me, me-
dullary groove; pr, primitive groove.
u
COMPARATIVE PHYSIOLOGY.
hypoblast, and intermediate less closely arranged cells, termed,
from their position, mesoblast.
It will be noticed that all future growth of the embryo be-
gins axially, at least in the early stages of its development.
As the subsequent growth and advance of the embryo de-
pend on an abundant and suitable nutritive supply, we must
now turn to those arrangements which are temporary and of
subordinate importance, but still for the time essential to devel-
opment.
THE EMBRYONIC MEMBRANES OF BIRDS.
It will be borne in mind throughout that the chief food-sup-
ply for the embryo bird is derived from the yelk ; and, as would
—pp
■vf.
Vt
w
Pig. 73.
Figs. 73-?5.— A series of diagrams intended to facilitate the comprehension of the
relations of the membranes to other parts (after Foster and Balfour). A, B, C, D,
E, F are vertical sections in the long axis of the embryo at different periods show-
ing the stages of development of the amnion and of the yelk-sac. I, II, III, IV
are transverse sections at about the same stages of development, i, ii, iii, pos-
terior part of longitudinal section, to illustrate three stages in formation of the
allantois. e, embryo; y, yelk; pp, pleuroperitoneal cavity; vt, vitelline mem-
brane of amniotic fold;'«/, allantois; a, amnion; a', alimentary canal.
be expected, the older the embryo the smaller the yelk, or, as it
is now called when limited by the embryonic membranes, the
REPRODUCTION,
75
\\ m
n
76
COMPARATIVE PHYSIOLOGY.
yelk-sac {umbilical vesicle of the mammalian embryo). The
manner in which this takes place will appear npon an inspec-
tion of the accompanying figures.
Very early in the history of the embryo two eminences, the
head and the tail folds, arise, and, curving over toward each
MO.
Fig. 70. — Diagrammatic longitudinal section through the axis of an embryo chick
(after Foster and Balfour). N. C, Neural canal; Ch, notochord; Fg, foregnt;
F. So, somatopleure; F. Sp, splanchnopleure; Sp, splanchnopleure, forming lower
wall of foregut; lit, heart; pp, pleuroperitoneal cavity; Am, amniotic fold; E,
epiblast; M, mesoblast; H, hypoblast.
other, meet after being joined by corresponding lateral folds.
Fusion and absorption result at this meeting-point, in the
inclosure of one cavity and the blending of two others. These
folds constitute the amniotic membranes, the inner of which
Fig. 77.— Diagrammatic longitudinal section of a chick of the fourth day (after Allen
Thomson), ep, epiblast; hy, hypoblast; sm, somatopleure; vm, splanchnopleure;
of, pf, folds of the amnion; pp, pleuroperitoneal cavity; am, cavity of the am-
nion'; at. allantois; a, position of the future anus; h, heart; i, intestine; vi, vitel-
line duct; ye, yelk; x, foregut; m, position of the mouth; me, mesentery.
forms the true amnion, the outer the false amnion (serous mem-
brane, subzonal membrane). Within the amnion proper is the
amniotic cavity filled with fluid (liquor amnii), while the space
between the true and false amniotic folds, which gradually in-
REPRODUCTION.
77
creases in size as the yelk-sac diminishes, forms the pleuro-
peritoneal cavity, body cavity, or coclom. The amniotic cavity
also extends, so that the embryo is surrounded by it or lies
centrally within it. The enlargement of the ccelom and exten-
sion of the false amniotic folds lead finally to a similar meeting
and fusion like that which occurred in the formation of the true
amniotic cavity. The yelk-sac, gradually lessening, is at last
withdrawn into the body of the embryo.
Fig. 76 shows how the amniotic head fold arises, from a
budding out of the epiblast and mesb blast at a point where the
original cell layers of the embryo have separated into two folds,
the somatopleure or body fold and the splanchnoplenre or vis-
ceral fold, owing to a division or cleavage of the mesoblast
toward the long axis of the body. Remembering this, it is
always easy to determine by a diagram the composition of any
one of the membranes or
folds of the embryo, for
the components must be
epiblast, mesoblast, or
hypoblast ; thus, the
splanclmopleure is made
up of hypoblast internally
and mesoblast externally
— a principle of great sig-
nificance, since, as will be
learned later, all the tis-
sues of the body may be
classified simply, and at
the same time scientifi-
cally, according to their
embryological origin.
The allantois is a
structure of much physi-
ological importance. It
arises at the same time as
the amniotic folds are
forming, by a budding or protrusion of the hind-gut into the
pleuro-peritoueal cavity, and hence consists of an outgrowth of
mesoblast lined by hypoblast.
Fig. 78. — Diagrammatic longitudinal section
through the egg of a fowl (after Duval).
al, cavity of allantois; alb. albumen; a/'t. mes-
enteron;" am, cavity of amnion; emb, embryo;
sh. egg-shell; v. m, vitelline membrane.
COMPARATIVE PHYSIOLOGY.
THE FCETAL (EMBRYONIC) MEMBRANES OF
MAMMALS.
The differences between the development of the egg mem-
branes of mammals and birds are chiefly such as result from
the absence in the
former of an egg-shell
and its membranes, and
of yelk and albumen.
The mammalian ovum
is inclosed by a zona
radiata {zona pelluci-
da) surrounded by an-
other very delicate cov-
ering (vitelline mem-
brane).
The growth of the
blastodermic vesicle
(yelk-sac) is rapid, and,
being filled with fluid,
the zona is thinned and
soon disappears.
The germinal area
alone is made up of
three layers of cells
(Fig. 100), the rest of
the upper part of the
oosperm being lined
with epiblast and hyp-
oblast, while the low-
er zone of the yelk-sac consists of epiblast only.
Simple, non-vascular villi, serving to attach the embryo
to the uterine walls, usually project from the epiblast of the
subzonal membrane. In the rabbit they do not occur every-
where, but only in that region of the epiblast beneath which
the mesoblast does not extend, with the exception of a patch
which soon appears and demarkates the site of the future pla-
centa. The amnion and allantois are formed in much the same
way as has been described for the chick.
At about the same period as these events are transpiring the
vascular yelk-sac has become smaller, and the allantois with
its abundant supply of blood-vessels is becoming more promi-
Fig. 79. — Diagrammatic longitudinal section
oosperm of rabbit at an advanced stage of preg-
nancy (KOlliker, after Bischoff). a, amnion; al,
allantois with its blood-vessels; e, embryo ; ds,
yelk-sac ; ed, ed' , ed", hypoblastic epithelium of
the yelk-sac and its stalk (umbilical vesicle and
cord); fd, vascular mesoblastic membrane of the
umbilical cord and vesicle ; pi, placental villi
formed by the allantois and subzonal membrane;
r, space filled with fluid between the amnion,
the allantois. and the yelk-sac ; ft, sinus termi-
nalis (marginal vitelline blood-vessel); u, urach-
us, or stalk of the allantois.
REPRODUCTION.
79
^4-^ro'/n*
nent, and extending between the amnion and subzonal mem-
brane.
The formation of the chorion marks an important step in
the development of mammals in which it plays an important
functional part. It is
the result of the fusion
of the allantois, which
is highly vascular,
with the subzonal
membrane, the villi of
which now become
themselves vascular
and more complex in
other respects.
An interesting re-
semblance to birds has
been observed (by Os-
born) in the opossum
(Fig. 83). When the
allantois is small the
blastodermic vesicle
(yelk-sac) has vascular
villi, which in all prob-
ability not only serve
the purpose of attach-
ing the embryo to the
uterine wall but derive
nourishment, not as in
birds, from the albumen of the ovum, but directly in some way
from the uterine wall of the mother. It will be remembered
that the opossum ranks low in the mammalian scale, so that this
resemblance is the more significant from an evolutionary point
of view.
The term chorion is now restricted to those regions of the
subzonal membrane to which either the yelk-sac or the allan-
tois is attached. The former zone has been distinguished as the
false chorion and the latter as the true chorion. In the rabbit
the false chorion is very large (Fig. 79), and the true (placen-
tal) chorion very small in comparison, but the reverse is the
case in most mammals. It will be noted that in both birds
and mammals the allantois is a nutritive organ. Usually
the more prominent and persistent the yelk-sac, the less so
Fig. 80.— Diagrammatic dorsal view of an embryo rab-
bit with its membranes at the stage of nine so-
mites (Hadclon, after Van Beneden and Julin).
aly allantois, showing from behind the tail fold
of the embryo; am, anterior border of true am-
nion ; a. v, area vasculosa, the outer border of
which indicates the farthest extension of the
mesoblast; hi, blastoderm, here consisting only of
epiblast and hypoblast; o. m. v, omphalomesen-
teric or vitelline veins ; p. am, proamnion ; pi,
non-vascular epiblastic villi of the future placen-
ta ; s. t, sinus terminalis.
80
COMPARATIVE PHYSIOLOGY.
the allantois, and vice versa ; they are plainly supplementary
organs.
The Allantoic Cavity.— The degree to which the various em-
bryonic membranes fuse together is very variable for different
groups of mammals, including our domestic species.
In ruminants, but especially in solipeds, the allantois as it
grows spreads itself over the inner surface of the subzonal
'Mm-
Fig. 81.— Embryo of dog. twenty-five days old. opened on the ventral side. Chest
and ventral walls have been removed, a, nose-pits; b, eyes; c, nnder-jaw (first
gill-arch); d, second gill-arch; e,f, 0, h, heart (e, right,/, left auricle; g, right, h,
left ventricle); i, aorta (origin of); kk, liver (in the middle between the two lobes
is the cut yelk-vein); /, stomach; m, intestine; n, yelk-sac; o, primitive kidneys;
p, allantois; q, fore-limbs; h, hind-limbs. The crooked embryo has been stretched
straight. (Ilaeckel, after Bischoff.)
membrane, often spoken of as the " chorion," while it also
covers, though capable of easy detachment, the outer surface of
the amnion ; and thus is formed the allantoic cavity. The por-
tion of the allantois remaining finally within the foetus becomes
the bladder, which during embryonic life communicates by its
contracted portion (urachus) with the general amniotic cavity.
REPRODUCTION.
81
Fig. 82. — Diagram of an embryo showing the relations of the vascular allantois to the
villi of the chorion (Cadiat). e, embryo lying in the cavity of the amnion; y$,
yelk-sac; al, allantois; A.Y, allantoic vessels dipping into the villi of the chorion;
ch. chorion.
-\-aro.
In the mare especially these parts can be readily distin-
guished. From the connection of the portion that ultimately
forms the bladder with the
main sac, as indicated
above, there is ground for
regarding the allantoic
fluid in the later stages of
gestation, at all events, as
a sort of urine.
This fluid is at an ear-
ly period abundant and
colorless, later yellowish,
and finally brown. Since
at one time it contains
albumen and sugar, it
may serve some purpose
in the nutrition of the
foetus.
When most suggestive
of urine in the latest stages
of gestation, it contains
6
Fig. 83.— Diagram of the foetal membranes of
the Virginian opossum (Haddon, after Os-
born). Two villi are shown greatly enlarged.
The processes of the cells, which have been
exaggerated, doubtless correspond to the
pseudopodia described by Caldwell, al,
allantois; am, amnion: s.t, sinus termi-
nalis; <«. z, subzonal membrane; r. villi on
the subzonal membrane in the region of
the yelk-sac ; ijs. yelk-sac. The vascular
splanchuopleure (hypoblast and mesoblast)
is indicated by the "black line.
82
COMPARATIVE PHYSIOLOGY.
a characteristic body, allantoin, related to uric acid, urea,
etc.
Certain bodies, being probably inspissated allantoic fluid,
have been termed " hippomanes. " They may either float free
in the fluid or be attached to the allantois by a slender pedicle.
The relation of the parts described above will become clearer
after a study of the accompanying cuts and those of preceding
pages, in which the allantois is figured.
Fig. 84.— Exterior of chorial sac; mare. (Chauveau.) A, body; B. C. cornua.
The Placenta. — This structure, which varies greatly in com-
plexity, may be regarded as the result of the union of structures
existing for a longer or shorter period, free and largely inde-
pendent of each other. With evolution there is differentiation
and complication, so that the placenta usually marks the site
where structures have met and fused, differentiating a new or-
gan; while corresponding atrophy, obliteration, and fusion take
place in other regions.
All placentas are highly vascular, all are villous, all dis-
charge similar functions in providing the embryo with nourish-
ment and eliminating the waste of its cell-life (metabolism).
In structural details they are so different that classifications of
mammals have been founded upon their resemblances and dif-
ferences. They will now be briefly described.
In marsupials the yelk-sac is both large and vascular; the
allantois small but vascular; the former is said (Owen) to be
attached to the subzonal membrane, the latter not; but no villi,
and consequently no true chorion, is developed. All mammals
REPRODUCTION.
83
Pig. 85. — Foetus of mare with its envelopes. (Chauveau.) A. chorion: C, amnion re-
moved from allantoicl cavity and opened to expose foetus; D, infundibulum of
urachus; B, allantoid portion of umbilical cord.
other than the monotremes and marsupials have a true allan-
toic placenta.
The Discoidal Placenta. — This form of placenta is that exist-
ing in the rodentia, insectivora, and cheiroptera. The condition
found in the rabhit is that which has been most studied. The
relation of parts is shown in Fig;. 79.
The uterus of the rodent is two-homed ; so we find in gen-
eral several embryos in each horn in the pregnant rabbit.
They are functionally independent, each having its own set of
84
COMPARATIVE PHYSIOLOGY.
membranes. It will be observed from the figure tbat tbe true
villous cborion is confined to a comparatively small region;
tbere is, however, in addition a false cborion witbout villi, but
bigbly vascular. This blending of forms of placentation wbicb
exist separately in different groups of animals is significant.
In tbe rabbit at a later stage tbere is considerable interming-
ling of foetal and maternal parts,,
i %~ Series of diagrams representing the relations of the decidna to the ovum, at
different periods, in the human subject. The decidua are dark, the ovum shaded
transversely. In 4 and 5 the chorionic vascular processes are figured (after Dal-
ton), 1. Ovum resting on the decidua serotina; 2. Decidua reflexa growing round
th'- ovum; 3. Completion of the decidua around the ovum; 4. Villi, growing out
all around the chorion; 5. The villi, specially developed at the site of the future
placenta, having atrophied elsewhere.
The Metadiscoidal Placenta.— This type, which, in general
naked-eye appearances, greatly resembles the former, is found
in man and the apes. The condition of things in man is by no
means as well understood as in the lower mammals, especially
in the early stages ; so that, while the following account is that
REPRODUCTION.
85
usually given in works on embryology, the student may as well
understand that our knowledge of human embryology in the
very earliest stages is incomplete and partly conjectural. The
reason of this is obvious : specimens for examination depending
on accidents giving rise to abortion or sudden death, often not
reaching the laboratory in a condition permitting of trust-
worthy inferences.
It is definitely known that the. ovum, which is usually fer-
tilized in the oviduct (Fallopian tube), on entering the uterus
becomes adherent to its wall and encapsuled. The mucous
membrane of the uterus is known to undergo changes, its com-
ponent parts increasing by cell multiplication, becoming in-
tensely vascular and functionally more active. The general
mucous surface shares in this, and is termed the decidua vera ;
but the locality where the ovum lodges is the seat of the great-
est manifestation of exalted activity, and is termed the decidua
serotina ; while the part believed to have invested the ovum by
Fig. 87.— Vascular system of the hnman frettis, represented diagrammatical]}- (TTnx-
ley). 77. heart: TA, aortic trunk: c, common carotid artery: r'. external carotid
artery: c". internal carotid artery: a, subclavian artery: v, vertebral artery: 1. '.?.
3. 4. 5. aortic arches: A', dorsal aorta: o. omphalo-mesenterie artery; <h\ vitelline
duct: r>'. omphalo-mesenterie vein; v1, umbilical vesicle; vp, portal vein: L. liver;
a. v. umbilical arteries: u", v". their endincrs in the placenta: '/'. umbilical vein;
Dr. ductus venosus; rh. hepatic vein: rr. inferior vena cava: vit, iliac veins; az,
vena azygos; vc', posterior cardinal vein; DC, duct of Cuvier; P. lung.
86
COMPARATIVE PHYSIOLOGY.
fused growths from, the junction of the decidua vera and sero-
tina is the decidua reflexa.
The decidua serotina and reflexa thus become the outermost
of all the coverings of the ovum. These and some other devel-
opments are figured below. It is to be remembered, however,
that they are highly diagrammatic, and represent a mixture of
inferences based, some of them, on actual observation and others
on analogy, etc.
The figures will convey some information, though appear-
ances in all such cases must be interpreted cautiously for the
reasons already mentioned.
During the first fourteen days villi appear over the whole
surface of the ovum ; about this fact there is no doubt. At
the end of the first month of fcetal life, a complete chorion
has been formed, owing, it would seem, to the growth of the
allantois (its mesoblast only) beneath the whole surface of the
subzonal membrane. From the chorionic surface vascular pro-
cesses clothed with epithelium project like the plush of velvet.
The allantois is compressed and devoid of a cavity, but abun-
dantly supplied with blood-vessels by the allantoic arteries and
veins, which of course terminate in capillaries in the villi.
Compare the whole series of figures.
Fig. 88.— Human ova during early stages of development. A and B, front and side
view of an ovum supposed to be about thirteen days old; e. embryonic area
(Quain, after Reichert); C, ovrnn of four to five weeks, showing the general
structure of the ovum before formation of the placenta. Partof the wall or me
ovum is removed to show the embryo in position (after Allen Thomson).
At this stage the condition of the chorion suggests the type
of the diffuse placenta which is normal for certain groups of
animals, as will presently be learned.
The subsequent changes are much better understood, for
parts are in general no longer microscopic but of considerable
size, and their real structure less readily obscured or obliterated.
The amniotic cavity continues to enlarge by growth of the
walls of the amnion and is kept filled with a fluid; the yelk-sac
REPRODUCTION.
87
is now very small ; the decidua renexa becomes almost non-
vascular, and fuses finally with the decidua vera and the cho-
rion, which except at one part has ceased to be villous and vas-
cular ; so that becoming thinner and thinner with the advance
of pregnancy, the single membrane, arising practically from
I d
Fig. 89.— Human embryo, twelve weeks old, with its coverings; natural size. The
navel-cord passes from the navel to the placenta, b, amnion; c, chorion; d, pla-
centa; d', remains of tufts on the smooth chorion; /. decidua reflexa (inner); g,
decidua vera (outer). (Haeckel after Bernhard Schu'ltze.)
this fusion of several, is of a low type of structure, the result of
gradual degeneration, as the role they once played was taken
up by the other parts.
But of paramount importance is the formation of the pla-
centa. The chorion ceases to be vascular except at the spot at
which the villi not only remain, but become more vascular and
branch into arborescent forms of considerable complexity. It
is discoidal in form, made up of a fcetal part just described and
a maternal part, the decidua serotina, the two becoming blended
COMPARATIVE PHYSIOLOGY.
so that the removal of one involves that of more or less of the
others. The connection of parts is far closer than that described
Fig. 90.— Diagram illustrating the decidua, placenta, etc. (after Liegeois). e, embryo;
i, intestine: p, pedicle of the umbilical vesicle; u. v, umbilical vesicle; a, amnion;
eh, chorion; v. t, vascular tufts of the chorion, constituting the fcetal portion of
the placenta; m.p, maternal portion of the placenta; d. v, decidua vera; d. r, de-
cidua reflexa; al, allantois.
for the rabbit ; and, even with the preparation that Nature
makes for the final separation of the placenta from both foetus
and mother, this event does not take place without some rupture
of vessels and consequent haemorrhage.
It is difficult to conceive of the great vascularity of the
human placenta without an actual examination of this structure
itself, which can be done after being cast off to great advan-
tage when floating in water ; by which simple method also the
thinness and other characteristics of the membranes can be
well made out.
The great vessels conveying the fcetal blood to and from the
REPRODUCTION. 89
placenta are reduced to three, two arteries and one vein. The
villi of the placenta (chorion) are usually said to hang freely
in the blood of the large irregular sinuses of the decidua sero-
tina; but this is so unlike what prevails in other groups of
animals that we can not refrain from believing that the state-
ment is not wholly true.
The Zonary Placenta. — In this type the placenta is formed
along a broad equatorial belt, leaving the poles free. This form
of placentation is exemplified in the carnivora, hyrax, the ele-
phant, etc.
In the dog, for example, the yelk-sac is large, vascular, does
not fuse with the chorion, and persists throughout. A rudi-
mentary discoid placenta is first formed, as in the rabbit ; this
gradually spreads over the whole central area, till only the ex-
tremes (poles) of the ovum remain free ; villi appear, fitting into
pits in the uterine surface, the maternal and foetal parts of the
placenta becoming highly vascular and closely approximated.
The chorionic zone remains wider than the placental. As in
man there is at birth a separation of the maternal as well as
foetal part of the placenta — i. e., the latter is deciduate; there is
also the beginning of a decidua reflexa.
The Diffuse Placenta. — As found in the horse, pig, lemur,
etc., the allantois completely incloses the embryo, and it be-
comes villous in all parts, except a small area at each pole.
The Polycotyledonary Placenta. — This form is that met with
in ruminants, in wThich case the allantois completely covers the
surface of the subzonal membrane, the placental villi being
gathered into patches (chorial cotyledons), which are equivalent
to so many independent placentas. The component villi fit into
corresponding pits in the uterine wall (uterine cotyledons),
which is specially thickened at these points. When examined
in a fresh condition, under water, they constitute very beautiful
objects. The pits referred to above into which the foetal villi fit
are, as shown in the figures on page 91, essentially the same in
structure as the villi themselves. In the cow the uterine cotyle-
dons are convex ; but in the sheep and goat they are raised con-
cave cups in which the openings for the foetal villi may be seen
with the naked eye. The differences are not essential ones.
Between the uterine cotyledons and the foetal villi which
fit into them a thickish, milky-looking fluid is found, the
" uterine milk " elaborated, no doubt, by the cells which line the
cotyledonous pits.
90 COMPARATIVE PHYSIOLOGY.
The placentation of certain of our domestic animals may be
thus expressed in tabular form (Fleming-) :
Simple placenta, j General- 1 s™- '
( Local and circular, i Bitch.
( Cow.
Multiple placenta. -j Sheep.
Comparing- the formation, complete development, and atro-
phy (in some cases) of the various foetal appendages in mam-
mals, one can not but perceive a common plan of structure,
with variations in the preponderance of one part over another
here arid there throughout. In birds these structures are sim-
pler, chiefly because less blended and because of the presence
of much food-yelk, albumen, egg-shell, etc., on the one hand,
and the absence of a uterine wall, with which in the mammal
the membranes are brought into close relationship, on the other ;
but, as will be shown later, whatever the variations, they are
adaptations to meet common needs and subserve common ends.
MICROSCOPIC STRUCTURE OF THE PLACENTA.
This varies somewhat for different forms, though, in that
there is a supporting matrix, minute (capillary) blood-vessels,
and epithelial coverings in the foetal and maternal surfaces, the
several forms agree.
The pig possesses the simplest form of placenta yet known.
The villi fit into depressions or crypts in the maternal uterine
mucous membrane. The villi, consisting of a core of connective
tissue, in which capillaries abound, are covered with a flat epi-
thelium; the maternal crypts correspond, being composed of
a similar matrix, lined with epithelium and permeated by
capillary vessels, which constitute a plexus or mesh-work. It
thus results that two layers of epithelium intervene between
the maternal and foetal capillaries.
The arrangement is substantially the same in the diffuse and
the cotyledonary placenta.
In the deciduate placenta, naturally, there is greater compli-
cation.
In certain forms, as in the fox and cat, the maternal tissue
shows a system of trabecular assuming a meshed form, in
which run dilated capillaries. These, which are covered with
REPRODUCTION.
91
a somewhat columnar epithelium, are everywhere in contact
with the foetal villi, which are themselves covered with a flat
epithelium.
Fig. 94.
Figs. 91 to 97. — Diagrammatic representation of the minute structure of the placenta
(Foster and Balfour, after Turner). F, foetal; M, maternal placenta; e, epithelium
of chorion; e\ epithelium of maternal placenta; d, foetal blood-vessels; d', mater-
nal blood-vessels; v, villus.
Fig. 91. — Placenta in most generalized form.
Fig. 92.— Structure of placenta of a pig.
Fiu. 93.— Of a cow.
Fig. 94.— Of a fox.
Fig. 95.— Of a cat.
In the case of the sloth, with a more discoidal placenta, the
dilatation of capillaries and the modification of epithelium are
greater.
92
COMPARATIVE PHYSIOLOGY.
In the placenta of the
apes and of the human sub-
ject the most marked depart-
ure from simplicity is found.
The maternal vessels are said
to constitute large intercom-
municating sinuses ; the villi
may hang freely suspended in
these sinuses, or be anchored
to their walls by strands of
tissue. There is believed to
be only one layer of epithe-
lial cells between the vessels
of mother and foetus in the
later stages of pregnancy.
This, while closely investing
Fig. 96.— Placenta of a sloth. Flat maternal the f oetal vessels (capillaries),
epithelial cells shown in position on ... , , , ,, ,
right side; on left they are removed and really belongs to the mater-
c£apu^ie^aexposedV.eSSel wUh itB bl°°d" nal structures. The signifi-
e-F.
Fig 97.— Structure of human placenta; ds, decidua serotina; L trabecnlae of serotina
passing to f«'lal villi; ca, curling artery; up, utero-placental vein; a;, prolongation
of maternal tissue on exterior of villus, outside cellular layer e', which may repre-
sent either endothelium of maternal blood-vessels or delicate connective tissue of
the serotina or both; e', maternal cells of the serotina.
REPRODUCTION. 93
ca'nce of this general arrangement will be explained in the
chapter on the physiological aspects of the subject.
It remains to inquire into the relation of these forms to one
another from a phylogenetic (derivative) point of view, or to
trace the evolution of the placenta.
Evolution. — Passing by the lowest mammals, in which the
placental relations are as yet imperfectly understood, it seems
clear that the simplest condition is found in the rodentia.
Thus, in the rabbit, as has been described, both yelk-sac and
allantois take a nutritive part ; but the latter remains small.
In forms above the rodents, the allantois assumes more and
more importance, becomes larger, and sooner or later predomi-
nates over the yelk-sac.
The discoidal, zonary, cotyledonary, etc., are plainly evolu-
tions from the diffuse, for both differentiation of structure and
integration of parts are evident. Tbe human placenta seems
to have arisen from the diffuse form ; and it will be remembered
that it is at one period represented by the chorion with its villi
distributed universally.
The resemblance of the embryonic membranes at any early
stage in man and other mammals to those of birds certainly
suggests an evolution of some kind, though exactly along what
lines that has taken place it is difficult to determine with exact-
ness ; however, as before remarked, nearly all the complica-
tions of the higher forms arise by concentration and fusion, on
the one hand, and atrophy and disappearance of parts once
functionally active, on the other.
Summary. — The ovum is a typical cell ; unspecialized in most
directions, but so specialized as to evolve from itself compli-
cated structures of higher character. The segmentation of the
ovum is usually preceded by fertilization, or the union of the
nuclei of male and female cells, which is again preceded by the
extrusion of polar globules. In the early changes of the ovum,
including segmentation, periods of rest and activity alternate.
The method of segmentation has relation to the quantity and
arrangement of the food-yelk. Ova are divisible generally
into completely segmenting (holoblastic), and those that under-
go segmentation of only a part of their substance (meroblastic) ;
but the processes are fundamentally the same.
Provision is made for the nutrition, etc., of the ovum, when
fertilized (oosperm) by the formation of yelk-sac and allan-
tois; as development proceeds, one becomes more prominent
94 COMPARATIVE PHYSIOLOGY.
than the other. The allantois may fuse with adjacent mem-
branes and form at one part a condensed and hypertrophied
chorion (placenta), with corresponding1 atrophy elsewhere.
The arrangement of the placenta varies in different groups of
animals so constantly as to furnish a basis for classification.
Whatever the variations in the structure of the placenta, it is
always highly vascular ; its parts consist of villi fitting into
crypts in the maternal uterine membrane — both the villi and
the crypts being provided with capillaries supported by a con-
nective-tissue matrix covered externally by epithelium. The
placenta in its different forms would appear to have been
evolved from the diffuse type.
The peculiarities of the embryonic membranes in birds are
owing to the presence of a large food-yelk, egg-shell, and egg-
mernbranes ; but throughout, vertebrates follow in a common
line of development, the differences which separate them into
smaller and smaller groups appearing later and later. The
same may be said of the animal kingdom as a whole. This
seems to point clearly to a common origin with gradual diver-
gence of type.
THE DEVELOPMENT OF THE EMBRYO ITSELF.
We now turn to the development of the body of the animal
for which the structures we have been describing exist. It is
important, however, to remember that the development of
parts, though treated separately for the sake of convenience,
really goes on together to a certain extent; that new structures
do not appear suddenly but gradually ; and that the same law
applies to the disappearance of organs which are being super-
seded by others. To represent this completely would require
lengthy descriptions and an unlimited number of cuts; but
with the above caution it is hoped the student may be able to
avoid erroneous conceptions, and form in his own mind that
series of pictures which can not be well furnished in at least
the space we have to devote to the subject. But, better than
any abstract statements or pictorial representations, would be
the examination of a setting of eggs day by day during their
development under a hen. This is a very simple matter, and,
while the making and mounting of sections from hardened
specimens is valuable, it may require more time than the
student can spare ; but it is neither so valuable nor so easily ac-
complished as what we have indicated; for, while the lack of
sections made by the student may be made up in part by the
exhibition to him of a set of specimens permanently mounted
or even by plates, nothing can, in our opinion, take the place
of the examination of eggs as we have suggested. It prepares
for the study of the development of the mammal, and exhibits
the membranes in a simplicity, freshness, and beauty which
impart a knowledge that only such direct contact with nature
can supply. To proceed with great simplicity and very little
apparatus, one requires but a forceps, a glass dish or two, a
couple of watch-glasses, or a broad section-lifter (even a case-
knife will answer), some water, containing just enough salt to
be tasted, rendered lukewarm (blood-heat).
96
COMPARATIVE PHYSIOLOGY.
Holding the egg longitudinally, crack it across the center
transversely, gently ahd carefully pick away the shell and its
Fig. 98.— Various stages in the development of the frog from the egg (after Howes).
1. The segmenting ovum, showing first cleavage furrow. 2. Section of the above
at right angles to the furrow. 3. Same, on appearance of second furrow, viewed
slightly from above. 4. The latter seen from beneath. 5. The same, on appear-
ance of first horizontal furrow. 6. The same, seen from above. 7. Longitudinal
section of 6. 8 and 9. Two phases in segmentation, on appearance of fourth and
fifth furrows. 10. Longitudinal vertical section at a slightly later stage than the
above. 11. Later I stage. Upper pigmented pole dividing more rapidly than lower.
12. Later phase of 11. 13. Longitudinal vertical section of 12. 14. Segmenting
ovum at blastopore stage. 15. Longitudinal vertical section of same. 13 and IB
•a 10 (all others x 5). 10. Longitudinal vertical section of embryo at a stage later
than 14d x 10). nc, nucleus; c. c, cleavage cavity; eg, epiblast; 1. 1, yelk-Bearing
lower-layer cells; bl, blastopore; al, archentcron (mid-gut); hb, hypoblast; wis,
undifferentiated mesoblast; ch, notochord; n. a, neural (cerebrospinal) axis.
membranes, when the blastoderm may be seen floating upward,
as it always does. It should be well examined in position,
THE DEVELOPMENT OF THE EMBRYO ITSELF. 97
using a hand lens, though this is
knowledge ; in fact, if the exam-
ination goes no further than the
naked-eye appearances of a dozen
eggs, selecting one every twenty-
four hours during incubation,
when opened and the shell and
membranes well cleared away,
such a knowledge will be sup-
plied as can be obtained from no
books or lectures however good.
It will be, of course, understood
that the student approaches these
examinations with some ideas
gained from plates and previous
reading. The latter will furnish
a sort of biological pabulum on
which he may feed till he can
supply for himself a more natu-
ral and therefore more healthful
one. While these remarks apply
with a certain degree of force to
all the departments of physiolo-
gy, they are of special impor-
tance to aid the constructive fac-
ulty in building up correct no-
tions of the successive rapid
transformations that occur in
the development of a bird or
mammal.
Fig. 99 shows the embryo of
the bird at a very early period,
when already, however, some of
the main outlines of structure
are marked out. Development
in the fowl is so rapid that a few
days suffice to outline all the
principal organs of the body. In
the mammal the process is slow-
er, but in the main takes place in
the same fashion.
As the result of long and pa-
7
not essential to getting a fair
Fig. 99.— Embryo fowl 3 mm. long, of
about twenty-four hours, seen from
above. 1 x 39. (Haddon, after
K611iker.) Mn, union of the med-
ullary folds in the region of the
hind-brain; Pr, primitive streak;
Pz, parietal zone ; 7?/, posterior
portion of widely open neural
groove; i?/'. anterior part of neu-
ral groove ; Rw, neural ridge ; Stz.
trunk-zone ; rAf. anterior amni
otic fold ; rD, anterior umbilical
sinus showing through the blasto-
derm. His divides the embryonic
rudiment into a central trunk-zone
and a pair of lateral or parietal
zones.
98
COMPARATIVE PHYSIOLOGY.
tient observation, it is now settled that all the parts of the most
complicated organism arise from the three-layered blastoderm
previously figured ; every part may be traced back as arising in
one or other of these layers of cells — the epiblast, mesoblast, or
hypoblast. It frequently happens that an organ is made up of
cells derived from more than one layer. Structures may, ac-
cordingly, be classified as epiblastic, mesoblastic, or hypoblastic ;
for, when two strata of cells unite in the formation of any part,
one is always of subordinate importance to the other : thus the
digestive organs are made up of mesoblast as well as hypo-
blast, but the latter constitutes the essential secreting cell mech-
anism. As already indicated, the embryonic membranes are
also derived from the same source.
The epiblast gives rise to the skin and its appendages (hair,
nails, feathers, etc.), the whole of the nervous system, and the
chief parts of the organs of special sense.
The mesoblast originates the vascular system, the skeleton,
all forms of connective tissue including the framework of
glands, the muscles, and the epithelial (endothelial) structures
covering serous membranes.
The hypoblast furnishes the secreting cells of the digestive
tract and its appendages — as the liver and pancreas— the lining
epithelium of the lungs, and the cells of the secreting mucous
membranes of their framework of bronchial tubes.
It is difficult to overrate the importance of these morpholog-
ical generalizations for the physiologist ; for, once the origin of
an organ is known, its function and physiological relations gen-
erally may be predicted with considerable certainty. We shall
Pig. 100.— Transverse section through the medullary groove and half the blastoderm
of a chick of eighteen hours (Foster and Balfour). E, epiblast; M, mesoblast;
//, hypoblast; mf', medullary fold; my, medullary groove; ch, notochord.
endeavor to make this prominent in the future chapters of this
work.
Being prepared with these generalizations, we continue our
study of the development of the bird's embryo. Before the end
THE DEVELOPMENT OP THE EMBRYO ITSELF. 99
of the first twenty-four hours such an appearance as that repre-
sented in Fig. 100 is presented.
The mounds of cells forming the medullary folds are seen
coming in contact to form the medullary {neural) canal.
Fig. 101.— Transverse section of embryo chick at end of first day (after KOlliker). M,
mesoblast; H. hypoblast; m, medullary plate; E. epiblast; m. g, medullary groove;
m.f, medullary fold; ch, chorda dorsalis ; P, protovertebral plate; d. in. division
of mesoblast.
The notochord, marking out the future bony axis of the
body, may also be seen during the first day as a well-marked
linear extension, just beneath the medullary groove. The cleav-
nf fif
Fig. 102. — Transverse section of chick at end of second day (KOlliker). E, epiblast;
H, hypoblast; e. m, external plate of mesoblast dividing (cleavage of mesoblast):
m.f, medullary fold; m. rj, medullary groove; ao, aorta; p, pleuroperitoneal cavity;
P, protovertebral plate.
age of the mesoblast, resulting in the commencement of the
formation of somatopleure (body -fold) and the splanchnopleure
(visceral fold), is also an early and important event. These
give rise between them to the pleuro-peritoneal cavity . The
portions of mesoblast nearest the neural canal form masses (ver-
tebral plates) distinct from the thinner outer ones (lateral
plates). The vertebral plates, when distinctly marked off, as
represented in the figure, are termed the protovertebral (meso-
blastic somites), and represent the future vertebra? and the vol-
untary muscles of the trunk ; the former arising from the inner
subdivisions, and the latter from the outer (muscle-plates). It
will be understood that the protovertebra? are the results of
100
COMPARATIVE PHYSIOLOGY.
m.b
au.p,
a.p.
transverse division of the columns of mesoblast that formed the
vertebral plates.
Before the permanent vertebrae are formed, a reunion of the
original pro to vertebrae takes place as one cartilaginous pillar,
followed by a new segmen-
tation midway between the
original divisions.
It is thus seen that a
large number of structures
either appear or are clearly
outlined during the first
day of incubation : the
primitive streak, primitive
groove, medullary plates
and groove, the neural ca-
nal, the head-fold, the
cleavage of the mesoblast,
the protovertebrae. with
traces of the amnion and
area opaca.
During the second day
nearly all the remaining
important structures of the
chick are marked out, while
those that arose during the
first day have progressed.
Thus, the medullary folds
close ; there is an increase
in the number of protover-
tebrae ; the formation of a
tubular heart and the great
blood-vessels ; the appear-
ance of the Wolffian duct ;
the progress of the head re-
gion ; the appearance of the
three cerebral vesicles at the anterior extremity of the neural
canal ; the subdivision of the first cerebral vesicle into the optic
vesicles and the beginnings of the cerebrum ; the auditory pit
arising in the third cerebral vesicle (hind-brain) ; cranial flex-
ure commences ; both head and tail folds become more dis-
tinct ; the heart is not only formed, but its curvature becomes
more marked and rudiments of auricles arise ; while outside
Fig. 103. — Embryo of chick, between thirty
and thirty-six hours, viewed from above
as an opaque object (Foster and Balfour).
/. h, forebrain; m. b, midbrain; h. b, hind-
brain; op. v, optic vesicle; au. p, auditory
pit ; o.f. vitelline vein ; p. v, mesoblastic
somite'; m.f, line of function of medulla-
ry folds abovejmedullary canal ; s.r, sinus
raomboidalis; I, tail-fold; p.r, remains of
primitive groove ; a.p, area pellucida.
THE DEVELOPMENT OP THE EMBRYO ITSELF. 101
the embryo itself the circulation of the yelk-sac is established,
the allantois originates, and the amnion makes rapid progress.
It may be noticed that the cerebral vesicles, the optic vesi-
cles, and the auditory pit are all derived from the epiblastic
accumulations which occur in the anterior extremity of the
embryo ; and their early appearance is prophetic of their physi-
ological importance.
The heart, too, so essential for the nutrition of the embryo,
by distributing a constant blood-stream, is early formed, and
becomes functionally active. It arises beneath the hind-end of
the fore-gut, at the point of divergence of the folds of the
B __
Fig. 104. — Diagram representing under surface of an embryo rabbit of nine days and
three hours old, illustrating development of the heart (after Allen Thomson). A.
view of the entire embryo; B, an enlarged outline of the heart of A; C. later stage
of the development of B ; h h, ununited heart; aa, aorUe; vv, vitillme veins.
splanchnopleure, and so lies within the pleuro-peritoneal cav-
ity, and is derived from the mesoblast. At the beginning the
heart consists of two solid columns ununited in front at first ;
later, these fuse, in part, so that they have been compared
with an inverted Y, in which the heart itself would correspond
to the lower stem of the letter Q) and the great veins (vitel-
line) to its main limbs. The solid cords of mesoblast become
102
COMPARATIVE PHYSIOLOGY.
hollow prior to their coalescence, when the two tubes become
one.
The entire blood-vascular system originates in the mesoblast
of the area opaca especially ; at first appearing in isolated spots
Pro. 105.— Chick on third day, seen from beneath as a transparent object, the head
being turned to one side (Foster and Balfour), a', false amnion; a, amnion; OH,
cerebral hemisphere; FB, MB, HB, anterior, middle, and posterior cerebral vesi-
cles; OP, optic vesicle; ot, auditory vesicle; OfV, omphalo-mesentcric veins; lit,
heart; Ao, bulbus arteriosus; ch, notochord; Of.a, omphalomesenteric arteries;
Pv, proto vertebra;; x, point of divergence of the splanchnopleural folds; y, ter-
mination of the fore-gut, V.
which come together as actual vessels are formed. The student
who will pursue the plan of examining a series of incubating
eggs will be struck with the early rise and rapid progress of the
vascular system of the embryo, which takes, when complete,
such a form as is represented diagramatically in Fig. 109.
The blood and the blood-vessels arise simultaneously from
THE DEVELOPMENT OF THE EMBRYO ITSELF. 103
the cells of the mesoblast by outgrowths of nuclear prolifera-
tion, and in the case of vessels (Fig. 143) extension of processes,
fusion, and excavation.
Fig. 106.— Diagram of the heart and principal arteries of the chick (Qnain). A repre-
sents an earlier and B and C later stages. 1, 1, omphalo-mesenteric reins; 2, auri-
cle; 3, ventricle; 4, aortic bulb; 5,5, primitive aortse; 6,6, omphalo-mesenteric
arteries; A, united aortse.
The fore-gut is formed by the union of the folds of the
splanchnopleure from before backward, and the hind-gut in a
similar manner by fusion from behind forward.
Fig. 107. — Diagrammatic outlines of the early arterial system of the mammalian em-
bryo (after Allen Thomson). A. At a period corresponding to the thirty-sixth or
thirty-eighth hour of incubation. B. Later stage, with two pairs of aortic arches.
/>. bulbus arteriosus of heart; v, vitelline arteries; 1 — 5, the aortic arches. The
dotted lines indicate the position of the future arches.
104
COMPARATIVE PHYSIOLOGY.
The excretory system is also foreshadowed at an early period
the Wolffian duct (Fig. 110), a mass of mesoblast cells near
which the cleavage of the mesoblast
takes place.
During the latter part of the sec-
ond day the vascular system, includ-
ing the heart, makes great progress.
The latter, in consequence of excessive
growth and the alteration of the rela-
tive position of other parts, becomes
bent up on itself, so that it presents
a curve to the right which represents
the venous part, and one to the left,
answering to the arterial. The rudi-
ments of the auricles also are to be
seen.
The arterial system is represented
at this stage by the expanded portion
of the heart known as the bulbus ar-
teriosus, and two extensions from it,
the aorta?, which, uniting above the
alimentary canal, form a single poste-
rior or dorsal aorta. From these great
arterial vessels the lesser ones arise,
and by subdivision constitute that
great mesh-work represented cliagram-
matically in Figs. 108, 109, from which
,. the course of the circulation may be
bryonic vascular system o-athered. The beating of the heart
(Wiedersheim). A, atrium; ° 0
A', A', dorsal aorta ; Ab, commences before the corpuscles nave
branchial vessels: Acd, cau- 1 , i -i ,-i„ . -i
dai artery; All, allantoic (hy- become numerous, and while the tub-
fSSFStSSfi l^bus ular system, through which the blood
arteriosus,- c, c>. external and js ^0 be driven, is still very incomplete.
internal carotids; D, ductus ' . *■
The events of the third day are of
Cuvieri (precaval veins); E,
external iliac arteries; H. G,
posterior cardinal vein ; Ic,
common iliac arteries; If. L.
Kill clefts ; R. A, right and
left, rootn of the aorta; S. S',
branchial collecting trunks
or veins ; ,S7>. subclavian ar-
tery ; 8b', subclavian vein;
Si, sinus venosus; V, ventri-
cle ; VC, anterior cardinal
vein; Vm, vitelline veins.
left side
the nature of the extension of parts
already marked out rather than the
formation of entirely new ones. The
following are the principal changes :
The bending of the head-end down-
ward (cranial flexure) ; the turning
of the embryo so that it lies on its
the completion of the vitelline circulation ; the in-
THE DEVELOPMENT OF THE EMBRYO ITSELF. 105
crease in the curvature of the heart and its complexity of struct-
ure by divisions ; the appearance of additional aortic arches
and of the cardinal veins ; the formation of four visceral clefts
and five visceral arches ; a series of progressive changes in
AAA
Fig. 109.— Diagram of circulation of yelk-sac at end of third day (Foster and Bal-
four). Blastoderm seen from below. Arteries made black. H, heart; AA, sec-
ond, third, and fourth aortic arches; A 0, dorsal aorta; L.of.A, left vitelline
artery; E.qf. A. right vitelline artery; 8. T. sinus terminalis; 'L. of, left vitelline
vein; R. of, right vitelline vein ; S. V, sinus venosus ; D. C, ductus
S. Ca. V, superior cardinal or jugular vein; V. Ca, inferior cardinal vein,
Cuvicri;
the organs of the special senses, such as the formation of the
lens of the eye and a secondary optic vesicle ; the closing in of
the optic vesicle ; and the formation of the nasal pits. In the
region of the future brain, the vesicles of the cerebral hemi-
spheres become distinct ; the hind-brain separates into cei'e-
106
COMPARATIVE PHYSIOLOGY.
! ;/p?^
Fio. 111.
Fio. 112.
^VaWFosTe^nTttX!^ through '^'^r remon of an embryo at end of fourth
aay i t osier and Balfour) n. c, neural canal; pr, posterior root of spinal nerve
A VT&of^hS^ r°0t-; A- °- °' !,l",erior ^y column of spinal cord;
notochor.i I v ,r U-T ln ^urse ot ■formation; m.p, muscle-plate; c.h,
vPin- w / w7f;rW0lhu'.' r$P,'- A0' dorsal aorta; »■«■«, Posterior cardina
vein; W. d, Wolffian duct; W. b, Wolffian body, consisting of tubules and Mat
THE DEVELOPMENT OF THE EMBRYO ITSELF. 107
pighian corpuscles; g. e, germinal epithelium; d, alimentary canal; M, commenc-
ing mesentery ; S. 0, somatopleure ; SP, splanchnopleure ; V, blood - vessels ;
pp, pleuroperitoneal cavity.
Fig. 111.— Diagram of portion of digestive tract of chick on fourth day (after GOtte).
The black line represents hypoblast; the shaded portion, mesoblast; Iff, lung di-
verticulum, expanding at bases into primary lung vesicle; st, stomach; /,hver;
p, pancreas.
Fig. 112.— Head of chick of third day, viewed sidewise as a transparent object (Hux-
ley). /«, cerebral hemispheres; lb, vesicle of third ventricle; II, mid-brain; III,
hind-brain; a, optic vesicle; cj, nasal pit; b, otic vesicle; d, infundibulum ; e,
pineal body ; h, notochord ; V, fifth nerve ; VII, seventh nerve ; VIII, united
glossopharyngeal and pneumogastric nerves. 1, 2, 3, 4, 5, the five visceral folds.
bellum and medulla oblongata ; the nerves, both cranial and
spinal, bud out from the nervous centers. The alimentary ca-
nal enlarges, a fore-gut and hind-gut being formed, the former
being divided into oesophagus, stomach, and duodenum ; the
latter into the large intestine and the cloaca. The lungs arise
from the alimentary canal in front of the stomach ; from simi-
lar diverticula from the duodenum, the liver and pancreas orig-
inate. Changes in the protovertebrae and muscle-plates con-
tinue, while the Wolffian bodies are formed and the Wolffian
duct modified.
Up to the third day the embryo lies mouth downward, but
now it comes to lie on its left side. See Fig. 105 with the ac-
companying description, it being borne in mind that the view is
from below, so that the right in the cut is the left in the em-
FrB.
Fig. 113.— Head of chick of fourth day. viewed from below as an opaque object (Fos-
ter and Balfour). The neck is cut across between third and fourth visceral folds.
C. //, cerebral hemispheres; F. B, vesicle of third ventricle: Op, eyeball; /if.
naso-frontal process; M, cavity of mouth; S. J/, superior maxillary process of F.
1. the first visceral fold (mandibular arch); F. 2, F. 3, second and third visceral
arches; JV, nasal pit.
bryo itself. Fig. 110 gives appearances furnished by a vertical
transverse section. The relations of the parts of the digestive
tract and the mode of origin of the lungs may be learned from
Fig. 111.
108
COMPARATIVE PHYSIOLOGY.
An examination of the figures and subjoined descriptions
must suffice to convey a general notion of the subsequent prog-
G.Ph
VII. rV.V. IF
MPr
Fig. 114.— Embryo at end of fourth day, seen as a transparent object (Foster and
Balfour). C'H, cerebral hemisphere; F. B. fore-brain, or vesicle of third ventricle
(thalamencephalon), with pineal gland (Pn) projecting; M. B, mid-brain; C.b,
cerebellum; IV. V, fourth ventricle; L, lens; cJw, choroid slit; Cen. V, auditory
vesicle; sm, superior maxillary process; IF, 2F, etc., first, second, etc., visceral
folds ; V, fifth nerve; VII. seventh nerve; O. Ph, glossopharyngeal nerve; Pg,
pneumogastric. The distribution of these nerves is also indicated ; ch, noto-
chord; lit, heart; MP. muscle-plates; W, wing; H. L, hind-limb. The amnion
has been removed. Al, allantois protruding from cut end of somatic stalk SS.
ress of the embryo. Special points will be considered, either in
a separate chapter now, or deferred for treatment in the body
of the work from time to time, as they seem to throw light
upon the subjects under discussion.
DEVELPOMENT OF THE VASCULAR SYSTEM IN VER-
TEBRATES.
This subject has been incidentally considered, but it is of
such importance morphological, physiological, and pathological,
as to deserve special treatment.
In the earliest stages of the circulation of a vertebrate the
arterial system is made up of a pair of arteries derived from the
single bulbus arteriosus of the heart, which, after passing for-
THE DEVELOPMENT OP THE EMBRYO ITSELF. 109
ward, bends round to the dorsal side of the pharynx, each giving
off at right angles to the yelk-sac a vitelline artery ; the aorta?
unite dorsally, then again separate and become lost in the pos-
terior end of the embryo. The so-called arches of the aorta are
large branches in the anterior end of the embryo derived from
the aorta itself.
The venous system corresponding to the above is composed
of anterior and posterior pairs of longitudinal (cardinal) veins,
the former (jugular, cardinal) uniting with the posterior to form
a common trunk (ductus Cuvieri) by which the venous blood is
returned to the heart. The blood from the posterior part of the
yelk-sac is collected by the vitelline veins, which terminate in
the median sinus venosus.
The Later Stages of the Foetal Circulation.— Corresponding
to the number of visceral arches five pans of aortic arches arise ;
but they do not exist together, the first two having undergone
more or less complete atrophy before the others appear. Figs.
115, 116 convey an idea of how the permanent forms (indicated by
darker shading) stand related to the entire system of vessels in
different groups of animals. Thus, in birds the right (fourth)
aortic arch only remains in connection with the aorta, the left
forming the subclavian artery, while the reverse occurs in
mammals. The fifth arch (pulmonary) always supplies the
lungs.
Fig. 115.— Diagrams of the aortic arches of mammal (Landois and Stirling, after
Ratlike). 1. Arterial trunk with one pair of arches, and an indication where the
second and third pairs will develop. 2. Ideal Btagc of five complete arches: the
fourth clefts are shown on the left side. 3. The two anterior pairs of arches have
disappeared. -1. Transition to the final stage. A, aortic arch; ad, dorsal aorta;
ax, subclavian or axillary artery; ft. external carotid; Ci. internal carotid; dB,
ductus arteriosus Botalli; P. pulmonary artery; iff, subclavian artery; la, truncua
arteriosus; v, vertebral artery.
The arrangement of the principal vessels in the bird, mam-
mal, etc. , is represented on page 110. In mammals the two prim-
110
COMPARATIVE PHYSIOLOGY.
itive anterior abdominal {allantoic) veins develop early and
unite in front with the vitelline : hut the right allantoic vein
and the right vitelline veins soon disappear, while the long com-
mon trunk of the allantoic and vitelline veins {ductus venosus)
passes through the liver, where it is said the ductus venosus
gives off and receives branches. The ductus venosus Arantii
persists throughout life. (Compare the various figures illustrat-
ing the circulation.)
A ... .. B
Fro. 116.— Diagram illustrating transformations of aortic arches in a lizard, A ; a
snake, B; a bird. C; a mammal, D. Seen from below, (lladdon, after Rathke.)
a, internal carotid; b, external carotid; c, common carotid. A. d, ductus Botalli
between the third and fourth arches; e, right aortic arch; /, subclavian; g, dorsal
aorta; h, left aortic arch; i, pulmonary artery; k, rudiment of the ductus Botalli
between the pulmonary artery and the aortic arches. B. d, right aortic arch; e,
vertebral artery; /, left aortic arch; A, pulmonary artery; i, ductus Botalli of the
latter. 0. it, origin of aorta; e, fourth arch of the right side (root of dorsal aorta);
/, right subclavian; g, dorsal aorta; h, left subclavian (fourth arch of the left
side); i, pulmonary artery; k and I, right and left ductus Botalli of the pulmonary
arteries. D. d, origin of aorto; e, fourth arch of the left side (root of dorsal
aorta); f, dorsal aorta; q, left vertebral artery; h, left subclavian; i, right sub-
clavian'(fourth arch of the right side); k. right vertebral artery; I, continuation of
the right subclavian; in. pulmonary artery; n, ductus Botalli of the latter (usually
termed ductus arteriosus).
With the development of the placenta the allantoic circula-
tion renders the vitelline subordinate, the vitelline and the larger
mesenteric vein forming the portal. The portal vein at a later
period joins one of the vena} advehentes of the allantoic vein.
At first the vena cava inferior and the ductus venosus enter
the heart as a common trunk. The ductus venosus Arantii be-
comes a small branch of the vena cava.
THE DEVELOPMENT OF THE EMBRYO ITSELF. \\\
The allantoic vein is finally represented in its degenerated
form as a solid cord {round ligament), the entire venous sup-
ply of the liver being derived from the portal vein.
The development of the heart has already been traced in the
fowl up to a certain point. In the mammal its origin and early
progress are similar and its further history may be gathered
from the following series of representations.
In the fowl the heart shows the commencement of a division
into a right and left half on the third day, and about the fourth
week in man, from which fact alone some idea may be gained
as to the relative rate of development. The division is effected
by the outgrowth of a septum from the ventral wall, which rap-
Fig. 118.
Fig. 117.
?!'•— Development of the heart in the human embryo, from the fourth to the
sixth week. A. hmbryo of four weeks (KOlliker, after Coste). B, anterior C
posterior views of the heart of an embryo of six weeks (KOlliker. after Eck'er)'
a upper limit of buccal cavity; c, buccal cavity; b, lies between the ventral ends
of second and third branchial arches; d, buds of upper limbs; e liver- f intes-
tine; 1, superior vena cava: 1', left superior vena cava; 1", opening of inferior
bu?b CaVa' aM 'eft anricles; 3- 3'- ri§ht and left ventricles; 4, aortic
Fig. 118.— Human embryo of about three weeks (Allen Thomson), uv. yelk-sac- al
allantois; am, amnion; ae, anterior extremity; pe. posterior extremity.
idly reaches the dorsal side, when the double ventricle thus
formed communicate by a right and a left auriculo-ventricular
opening with the large and as yet undivided auricle. Later an
incomplete septum forms similar divisions in the auricle ; the
aperture {foramen ovale) left by the imperfect growth of this
wall persisting throughout foetal life.
The Eustachian valve arises on the dorsal wall of the right
auricle, between the vena cava inferior and the right and left
112
COMPARATIVE PHYSIOLOGY.
venae cavaa superiores ; but in many mammals, among which is
man, the left vena cava superior disappears during fcetal life.
For the present we may simply say that the histories of the
development of the heart, the blood-vessels, and the blood itself
are closely related to each other, and to the nature and changes
of the various methods in which oxygen is supplied to the blood
and tissues, or, in other words, to the development of the respir-
atory system.
THE DEVELOPMENT OF THE UROGENITAL SYSTEM.
Without knowing the history of the organs, the anatomical
relations of parts with uses so unlike as reproduction on the one
hand and excretion on the other, can not be comprehended; nor,
as will be shortly made clear, the fact that the same part may
serve at one time to remove waste matters (urine) and at an-
other the generative elements.
The vertebrate excretory system may be divided into three
parts, which result from the differentiation of the primitive kid-
ney which has been effected during the slow and gradual evo-
lution of vertebrate forms :
1. The head-kidney (pronephros).
2. The Wolffian body (mesonephros).
3. The kidney proper, or metanephros.
But in this instance, as in others to some of which allusion
has already been made, these three parts are not functional at
the same time. The pronephros arises from the anterior part
of the segmental duct, pronephric duct, duct of primitive kid-
ney, and archinephric duct, and in the fowl is apparent on the
third day ; but the pronephros is best developed in the ichthy-
Pio. 110. — Diagrams illustrating development of pronephron in the fowl (Ilnddon).
ao, aorta; o.c, body-cavity; ep, epiblast with its epitrichial (flattened) layer; hy,
hypoblast; m. s, mesoblastic somite; n. c, neural canal; nch, notochord; p. I.., pro-
nephric tubule; so, somatic, and up, splanchnic mesoblaBt.
THE DEVELOPMENT OF THE EMBRYO ITSELF. H3
opsida (fishes and amphibians). A vascular process from the
peritoneum (glomerulus) projects into a dilated section of the
body cavity, which is in part separated from the rest of this
cavity (ccelom). This process, together with the segmental duct,
now coiled, and certain short tubes developed from the original
duct, make up the pronephros. The segmental duct opens at
length into the cloaca.
The mesonephros (Wolffian body), though largely developed
in all vertebrates during fcetal life, is not a persistent excretory
organ of adult life.
In the fowl recent investigation has shown that the Wolffian
(segmental) tubes originate from outgrowths of the Wolffian
Fig. 120.
Fig. 181.
Fig. 120. — Rudimentary primitive kidney of embryonic dog. The posterior portion of
the body of the embryo is seen from the ventral side, covered by the intestinal
layer of the yelk-sac. which has been torn away, and thrown back in front in
order to show the primitive kidney ducts with the primitive kidney tubes (a), h,
primitive vertebrae; c, dorsal medulla; d, passage into the pelvic intestinal cavity.
(Hacckel. alter Bischoff.)
Fig. 121. — Primitive kidney of a human embryo, v. the urine-tubes of the primitive
kidney: ir. Wolffian duct; w', upper end of the latter (Morgagni's hydatid*: m,
Mfillerian duct; m'. upper end of the latter (Fallopian hydatid): g, hermaphrodite
> gland. (After Kobelt.)
duct aud also from an intermediate cell-mass, from which latter
the Malpighian bodies take rise. The( tubes, at first not con-
114
COMPARATIVE PHYSIOLOGY.
nected with the duct, finally join it. This organ is continuous
with the pronephros ; in fact, all three (pronephros, rnesone-
phros, and metanephros) may be regarded as largely continua-
tions one of another.
The metanephros, or kidney proper, arises from mesoblast at
the posterior part of the Wolffian body. The ureter originates
Fig. 122.— Section of the intermediate ceil-mass of fourth day (Foster and Balfour,
after Waldeyer). 1 x 160. m. mesentery; L. somatopleure; «', portion of the
germinal epithelium from the duct of Miiller is formed by involution; a, thick-
ened portion of the germinal epithelium, in which the primitive ova, C and o. are
lying; E, modified mesoblast which will form the stroma of the ovary; WK,
Wolffian body; y, Wolffian duct.
first from the hinder portion of the Wolffian duct. In the fowl
the kidney tubules bud out from the m^eter as rounded eleva-
tions. The ureter loses its connection wTith the Wolffian duct
and opens independently into the cloaca.
The following account will apply especially to the higher
vertebrates :
The segmental (archinephric) duct is divided horizontally
into a dorsal or Wolffian (mesonephric) duct and a ventral or
Miillerian duct. The Wolffian duct, as we have seen, develops
into both ureter and kidney proper.
To carry the subject; somewhat further back, the epithelium
THE DEVELOPMENT OF THE EMBRYO ITSELF. 115
lining the coelom at one region becomes differentiated into col-
umnar cells {germinal epithelium) which by involution into
the underlying mesoblast forms a tubule extending from before
backward and in close relation with the Wolffian duct, thus
forming the Mullerian duct by the process of cleavage and
separation referred to previously.
Fig. 123. — Diagrammatic representation of the genital organs of a human embryo pre-
vious to sexual distinction (Allen Thomson!. TT". Wolffian body; gc, genital cord;
m, Mlillerian duct: w, Wolffian duct: ug, urogenital sinus; cp. clitoris or penis;
i, intestine; cl, cloaca; Is, part from which the scrotum or labia majora are devel-
oped; ot, origin of the ovary or testicle respectively; x. part of the Wolffian body
developed later into the cohi vasculosis 3, ureter: 4. bladder: 5, urachus.
The future of the Mullerian and Wolffian ducts varies ac-
cording to the sex of the embryo.
In the male the Wolffian duct persists as the vas deferens ;
in the female it remains as a rudiment in the region near the
ovary (hydatid of Morgagni). In the female the Mulleriau duct
becomes the oviduct and related parts (uterus and vagina) ; in
the male it atrophies. One, usually the right, also atrophies in
female birds. The sinus pocularis of the pi'ostate is the remnant
in the male of the fused tubes.
The various forms of the generative apparatus derived from
the Mullerian ducts, as determined by different degrees of fu-
116
COMPARATIVE PHYSIOLOGY.
Fig. 134. — Diagram of the mammalian type of male sexual organs (after Quain). Com-
pare with Figs. 123, 125. C. Cowper's gland of one side; cp, corpora cavernosa
penis, cut short; e, caput epididymis; g, gubernaculum; i, rectum; m, hydatid of
Morgagni, the persistent anterior end of the Miillerian duct, the conjoint posterior
ends of which form the uterus masculinus; pi', prostate gland; s, scrotum; sp,
corpus spongiosum urethra; t, testis (testicle) in the place of its original forma-
tion. The dotted line indicates the direction in which the testis and epididymis
change place in their descent from the abdomen into the scrotum; vd, vas defer-
ens; vh, vas aberrans; vs, vasicula seminalis; W, remnants of Wolffian body (the
organ of Giraldds or paradidymis of Waldeyer); 3, 4, 5, as in Fig. 125.
sion, etc., of parts, may be learned from the accompanying
figures.
n v C
Fio. 125.— Diagram of the mammalian type of female sexual organs (after Quain).
The dotted iines in one figure indicate functional organs in the other. C, gland of
Bartholin (OowperV gland); <■./:, corpus cavernosum clitoridis; dQ, remains of the
left Wolffian duct, which may persist as the duct of Gaertner; f, abdominal open-
ing of left Fallopian tube; ground ligament (corresponding tofhegubernaculum);
//.hymen; i, rectum; I, labium; m, cut Fallopian tube (oviduct, or Miillerian duct)
of the right wide; //, nyinpha; o, left ovary; po. parovarium; SO, vascular bulb or
corpus spongiosum; u, uterus; v, vulva; va, vagina; W, scattered remains of Wol-
ffian tubes (paroophoron); w, cut end of vanished right Wolffian duct; 3, ureter; 4,
•bladder passing below into tin.' urethra ; 5, urachus, or remnant, of stalk of allantois.
THE DEVELOPMENT OF THE EMBRYO ITSELF. Hf
In both sexes the most posterior portion of the Wolffian duct
gives rise to the metanephros, or what becomes the permanent
kidney and ureter ; in the male also to the vas deferens, testicle,
vas aberrans, and seminal vesicle.
The ovary has a similar origin to the testicle ; the germinal
epithelium furnishing the cells, which are transformed into
Graafian follicles, ova, etc., and the mesoblast the stroma in
which these structures are imbedded.
In the female the parovarium remains as the representative
of the atrophied Wolffian body and duct.
The bladder and urachus are both remnants of the formerly
extensive allantois. The final forms of the genito-urinary or-
gans arise by differentiation, fusion, and atrophy : thus, the
cloaca or common cavity of the genito-urinary ducts is divided
ALL,
ALL
Fig. 128. Fig. 129.
Figs. 126 to 129.— Diagrams illustrating the evolution of the posterior passages (after
Landois and Stirling).
Fig. 126. — Allantois continuous with rectum.
Pig. 127.— Cloaca formed.
Fig. 128. — Early condition in male, before the closure of the folds of the groove on
the posterior side of the penis.
Fig. 129. — Early female condition.
A, commencement of proctodeum; ALL, allantois; B. bladder; C. penis; CL, cloaca;
M, Miillerian duct; J?, rectum; U, urethra; S, vestibule; SU, urogenital sinus: 1'.
vas deferens in Fig. 128, vagina in Fig. 129.
by a septum (the perineum externally) into a genito-urinary
and an intestinal (anal) part ; the penis in the male and the
corresponding clitoris in the female appear in the region of the
cloaca, as outgrowths which are followed by extension of folds
of integument that become the scrotum in the one sex and the
labia in the other.
The urethra arises as a groove in the under surface of the
118
COMPARATIVE PHYSIOLOGY.
penis, which becomes a canal. The original opening1 of the
urethra was at the base of the penis.
In certain cases development of these parts is arrested at
various stages, from which result abnormalities frequently re-
quiring interference by the surgeon.
The accounts of the previous chapters do not complete the
history of development. Certain of the remaining subjects
that are of special interest, from a physiological point of view,
will be referred to again ; and in the mean time we shall con-
sider rather briefly some of the physiological problems of this
subject to which scant reference has as yet been made. Though
the physiology of reproduction is introduced here, so that ties
of natural connection may not be severed, it may very well be
omitted by the student who is dealing with embryology for the
Pig. 130.— Various forms of mammalian uteri. A. Ornithorhynchus. B. Didelphys
dorsigera. C. Phalangista vulpina. D. Double uterus and vagina; human anoma-
ly. E. Lepus cuniculus (rabbit), uterus duplex. F. Uterus bicornis. G. Uterus
bipartitus. H. Uterus simplex (human), a, anus; cl, cloaca; o. d, oviduct; o. I,
os tincse (os uteri); ov, ovary: r, rectum; s, vaginal septum; u. b, urinary bladder;
ur, ureter; ur. o, orifice of same; u. s, urogenital sinus; ut, uterus; v, vagina; v. c,
vaginal caecum (Haddon).
first time, and in any case should be read again after the other
functions of the body have been studied.
THE PHYSIOLOGICAL ASPECTS OF DEVELOPMENT.
According to that law of rhythm which, as we have seen,
prevails throughout the world of animated nature, there are
periods of growth and progress, of quietude and arrest of devel-
THE DEVELOPMENT OF THE EMBRYO ITSELF. H9
opment ; and in vertebrates one of the most pronounced epochs
— in fact, the most marked of all — is that by which the young
organism, through a series of rapid stages, attains to sexual ma-
turity.
While the growth and development of the generative organs
share to the greatest degree in this progress, other parts of the
body and the entire being participate.
So great is the change that it is common to indicate, in the
case of the human subject, the developed organism by a new
name — the " boy " becomes the " man," the " girl " the " woman.1'
Relatively this is by far the most rapid and general of all the
transformations the organism undergoes during its extra-uter-
ine life. In this the entire body takes part, but very unequally.
The increase in stature is not proportionate to the increase in
weight, and the latter is not so great as the change in form.
The modifications of the organism are localized and yet affect
the whole being. The outlines become more rounded ; the pel-
vis in females alters in shape ; not only do the generative organs
themselves rapidly undergo increased development, but certain
related glands (mammae) participate ; hair appears in certain
regions of the body ; the larynx, especially in the male, under-
goes enlargement and changes in the relative size of parts, re-
sulting in an alteration of voice (breaking of the voice), etc. —
all in conformity with that excess of nutritive energy which
marks this biological epoch.
Correlated with these physical changes are others belonging
to the intellectual and moral (psychic) nature equally impor-
tant, and, accordingly, the future being depends largely on the
full and un warped developments of these few years.
Sexual maturity, or the capacity to furnish ripe sexual ele-
ments (cells), is from the biological standpoint the most impor-
tant result of the onset of that period termed, as regards the
human species, puberty.
The age at which this epoch is reached varies with race,
sex, climate, and the moral influences which envelop the indi-
vidual. In temperate regions and with European races puberty
is reached at from about the thirteenth to the eighteenth year
in the female, and rather later in the male, in whom develop-
ment generally is somewhat slower. Changes analogous to the
above occur in all vertebrates. It is at this period that differ-
ences of form, voice, disposition, and other physical and psychic
characteristics first become pronounced.
120
COMPARATIVE PHYSIOLOGY.
As a matter of fact, the pig, sheep, goat, cat, dog, and certain
other animals may conceive when less than one year old ; and the
cow and the mare when under two years.
At such periods these animals are not of course mature and
should not be bred.
OVULATION.
In all vertebrates, at periods recurring with great regularity,
the generative organs of the female manifest unusual activity.
This is characterized by increased vascularity of the ovary and
adjacent parts ; with other changes dependent on this, and that
heightened nerve influence which, in the vertebrate, seems to be
inseparable from all important functional changes. Ovulation
is the maturation and discharge of ova from the Graafian folli-
cles. The latter, reaching the exterior zone of the ovaxy, be-
coming distended and thinned, burst externally and thus free
the ovum. The follicles being very vascular at this period,
blood escapes, owing to this rupture, into the emptied capsule
and clots ; and as a result of organization and
subsequent degeneration undergoes a certain
series of changes dependent on the condition
of the ovary and related organs, which varies
according as the ovum has been fertilized or
not. When fertilization occurs the Graafian
follicle undergoes changes of a more marked
and lasting character, becoming a true corpus
luteum of pregnancy.
The number of Graafian follicles that ma-
ture and the number of ripe ova that escape at
about the same period varies, of course, with
the species and the individual, and is not al-
ways the same in the latter.
In species that usually bear several young
at a birth a corresponding number of ova must
be ripened and fertilized at about the same
time ; while the reverse holds for those that
usually give birth to but one.
The ovum in the fowl is fertilized in the
upper part of the oviduct; in the mammal
mostly in this region also, as is shown by the
site of the embryos in those groups of animals with a two-
horned uterus, and the occasional occurrence of tubal pregnan-
Fio. 131.— Ovary of
rabbit at period
of oestrum, show-
ing various stages
of extniHion of
ova. (C'hauveau.)
THE DEVELOPMENT OF THE EMBRYO ITSELF. 121
cy in woman. But this is not, in the human subject at least,
invariably" the site of impregnation. After the ovum has been
set free, as above described, it is conveyed into the oviduct
(Fallopian tube), though exactly how is still a matter of dis-
pute : some holding that the current produced by the action
of the ciliated cells of the Fallopian tube suffices ; others that
the ovum is grasped by the fimbriated extremity of the tube as
part of a co-ordinated act. It is likely, as in so many other
instances, that both views are correct but partial; that is to
say, both these methods are employed. The columnar ciliated
cells, lining the oviduct, act so as to produce a current in the
direction of the uterus, thus assisting the ovum in its passage
toward its final resting place.
CEstrum. — As a part of the general activity occurring at this
time, the uterus manifests certain changes, chiefly in its inter-
nal mucous lining, in which thickening and increased vascular-
Fio. 132.— Diagram of the human uterus
just before menstruation. The shaded
portion represents the mucous mem-
brane (Hart and Barbour, after J.
Williams).
Fie 133.— Uterus after menstruation has
just ceased. The cavity of the body
of the uterus is supposed to have
been deprived of mucous membrane
(J. Williams).
ity are prominent. A flow of blood from the uterus in the form
of a gentle oozing follows; and as the superficial parts of the
mucous lining of the uterus undergo- softening and fatty degen-
122 COMPARATIVE PHYSIOLOGY.
eration, they are thrown off and renewed at these periods (cata-
vienia, menses, etc.), provided pregnancy does not take place.
In mammals helow man, in their natural state, pregnancy does
almost invariably take place at such times, hence this exalted
activity of the mucous coat of the uterus, in preparation for the
reception and nutrition of the ovum, is not often in vain. In
the human subject the menses appear monthly ; pregnancy may
or may not occur, and consequently there may be waste of na-
ture's forces ; though there is a certain amount of evidence that
menstruation does not wholly represent a loss; but that it is
largely of that character among a certain class of women is
only too evident. As can be readily understood, the catamenial
flow may take place prior to, during, or after the rupture of the
egg-capsule.
As the uterus is well supplied with glands, during this
period of increased functional activity of its lining membrane,
mucus in considerable excess over the usual quantity is dis-
charged ; and this phase of activity is continued for a time should
pregnancy occur.
All the parts of the generative organs are supplied with
muscular tissue, and with nerves as well as blood-vessels, so
that it is possible to understand how, by the influence of nerve-
centers, the various events of ovulation, menstruation, and
those that follow when pregnancy takes place, form a related
series, very regular in their succession, though little prominent
in the consciousness of the individual animal when normal.
In all animals, without exception, the disturbance of the
generative organs during the rutting season (oestrum) is accom-
panied by unusual excitement and special alterations in the
temper and disposition, while it may perhaps be said that the
whole organism is correspondingly affected.
The frequency of the season of heat or rutting is variable, as
also its duration. In most of the domestic animals it lasts but a
few days; though in the bitch it may be prolonged for a
month.
It is not common for conception to occur in the human sub-
ject while the young one is being suckled, and the same remark
applies to the domestic animals, though less so, and with con-
siderable variation for different species.
Naturally, the periods of oestrum will depend considerably
on the occurrence of impregnation and the duration of gesta-
tion. It is usual for the mare to be in season in spring and fall,
THE DEVELOPMENT OP THE EMBRYO ITSELF. 123
but, of course, if impregnated in the spring, there will be no au-
tumn oestrum on account of the prolonged period of gestation
in this instance ; and, similarly, in the case of the cow and other
animals.
It is important to recognize that rutting is only the evidence
of the maturation of the Graafian follicle within the ovary and
of correlated changes.
In a state of nature — i. e., in the case of wild animals — the
male experiences a period of sexual excitement corresponding
with an increased activity of the sexual organs and at periods
answering to the rutting season of the female. In some species
the testes descend into the scrotum only at this season. This
may be observed in the rabbit. But in our domestic animals, as
a class, the male, though capable of copulation at all times, is ex-
cited only by the presence of a female in season. It is only at
such periods that the approach of the male is permitted by the
opposite sex.
THE NUTRITION OF THE OVUM (OOSPERM).
This will be best understood if it be remembered that the
ovum is a cell, undifferentiated in most directions, and thus a
sort of amoeboid organism. In the fowl it is known that the
cells of the primitive germ devour, amoeba-like, the yelk-cells,
while in the mammalian oviduct the ovum is surrounded by
abundance of proteid, which is doubtless utilized in a somewhat
similar fashion, as also in the uterus itself, until the embryonic
membranes have formed. To speak of the ovum being nour-
ished by diffusion, and especially by osmosis, is an unnecessary
assumption, and, as we believe, at variance with fundamental
principles; for we doubt much whether any vital process is
one of pure osmosis. As soon as the yelk-sac and allantois
have been formed, nutriment is derived in great part through
the vessel-walls, which, it will be remembered, are differenti-
ated from the cells of the mesoblast, and, it may well be as-
sumed, have not at this early stage entirely lost their amoeboid
character. The blood-vessels certainly have a respiratory func-
tion, and suffice till the more complicated villi are formed.
The latter are in the main similar in structure to the villi of the
alimentary tract, and are adapted to being surrounded by sim-
ilar structures of maternal origin. Both the maternal crypts
and the foetal villi are, though complementary in shape, all but
124 COMPARATIVE PHYSIOLOGY.
identical in minute structure in most instances. In each case
the blood-vessels are covered superficially by cells which we
can not help thinking are essential in nutrition. The villi are
both nutritive and respiratory. It is no more difficult to under-
stand their function than that of the cells of the endoderm of a
polyp, or the epithelial coverings of lungs or gills.
Experiment proves that there is a respiratory interchange
of gases between the maternal and fcetal blood which nowhere
mingle physically. The same law holds in the respiration of
the foetus as in the mammals. Oxygen passes to the region
where there is least of it, and likewise carbonic anhydride. If
the mother be asphyxiated so is the foetus, and indeed more
rapidly than if its own umbilical vessels be tied, for the mater-
nal blood in the first instance abstracts the oxygen from that
of the foetus when the tension of this gas becomes lower in the
maternal than in the fcetal blood; the usual course of affairs
is reversed, and the mother satisfies the oxygen hunger of her
own blood and tissues by withdrawing that which she recently
supplied to the foetus. It will be seen, then, that the embryo is
from the first a parasite. This explains that exhaustion which
pregnancy, and especially a series of gestations, entails. True,
nature usually for the time meets the demand by an excess of
nutritive energy : hence many animals are never so vigorous in
appearance as when in this condition ; often, however, to be fol-
lowed by corresponding emaciation and senescence. The full
and frequent respirations, the bounding pulse, are succeeded by
reverse conditions ; action and reaction are alike present in the
animate and inanimate worlds. Moreover, it falls to the parent
to eliminate not only the waste of its own organism but that of
the foetus ; and not infrequently in the human subject the over-
wrought excretory organs, especially the kidneys, fail, entailing
disastrous consequences.
The digestive functions of the embryo are naturally inact-
ive, the blood being supplied with all its needful constituents
through the placenta by a much shorter process ; indeed, the
placental nutritive functions, so far as the foetus is concerned,
may be compared with the removal of already digested ma-
terial from the alimentary canal, though of course only in a
general way. During fcetal life the digestive glands are
developing, and at the time of birth all the digestive juices
are secreted in an efficient condition, though only relatively
so, necessitating a special liquid food (milk) in a form in which
THE DEVELOPMENT OP THE EMBRYO ITSELF. 125
all the constituents of a normal diet are provided, easy of diges-
tion.
Bile, inspissated and mixed with the dead and cast-off epi-
thelium of the alimentary tract, is abundant in the intestine at
birth ; but bile is to be regarded perhaps rather in the light of
an excretion than as a digestive fluid. The skin and kidneys,
though not functionless, are rendered unnecessary in great part
by the fact that waste can be and is withdrawn by the placenta,
which proves to be a nutritive, respiratory, and excretory organ ;
it is in itself a sort of abstract and brief chronicle of the whole
physiological story of foetal life.
All the foetal organs, especially the muscles, abound in an
animal starch (glycogen), which in some way, not well under-
stood, forms a reserve fund of nutritive energy which is pretty
well used up in the earlier months of pregnancy. We may sup-
pose that the amceboid cells — all the undifferentiated cells of
the body — feed on it in primitive fashion; and it will not be
forgotten that the older the cells become, the more do they de-
part from the simpler habits of their earlier, cruder existence ;
hence the disappearance of this substance in the later months of
foetal life.
In one respect the foetus closely resembles the adult : it draws
the pabulum for all its various tissues from blood which it-
self may, perhaps, be regarded as the first completed tissue. We
are, accordingly, led to inquire how this river of life is distrib-
uted ; in a word, into the nature of the foetal circulation.
Foetal Circulation. — The blood leaves the placenta by the
umbilical vein, reaches the inferior vena cava, either directly
(by the ductus venosus), or, after first passing to the liver (by
the venae advehentes, and returning by the venae, revehent.es),
and proceeds, mingled with the blood returning from the lower
extremities, to the right auricle. This blood, though far from
being as arterial in character as the blood after bii'th, is the best
that reaches the heart or any part of the organism. After arriv-
ing at the right auricle.being dammed back by the Eustachian
valve, it avoids the right ventricle, and shoots on into the left
auricle, passing thence into the left ventricle, from which it is
sent into the aorta, and is tben carried by the great trunks of this
arch to the head and iipper extremities. The blood returning
from these parts passes into the right auricle, then to the corre-
sponding ventricle, and thence into the pulmonary artery; but,
finding the branches of this vessel unopened, it takes the line of
Pulmonary Art
Foramen Ovale
Eustachian Valve.
Bight Auric. -Vent. Opening. fc\%l./i!. / *%.
Bladder
Pulmonary Art.
Left Auricle.
.Left Auric. ■ Vent.
Opening,
Hepatic Vein.
Branches of the
Umbilical Vein, .
to the Liver.
Ductus Venoms.
Internal Iliac Arteries.
Via. 13-1.— Diagram of the foetal circulation (Flint).
THE DEVELOPMENT OP THE EMBRYO ITSELF. 127
least resistance through the ductus arteriosus into the aortic
arch beyond the point "where its great branches emerge. It will
be seen that the blood going to the head and upper parts of the
body is greatly more valuable as nutritive pabulum than the rest,
especially in the quantity of oxygen it contains ; that the blood
of the foetus, at best, is relatively ill-supplied with its vital essen-
tial ; and as a result we find the upper (anterior in quadrupeds)
parts of the foetus best developed, and a decided resemblance be-
tween the mammalian foetus functionally and the adult forms of
reptiles and kindred groups of lower vertebrates. But this con-
dition is well enough adapted to the general ends to be attained
at this period — the nourishment of structures on the way to a
higher path of progress.
As embryonic maturity is being reached, preparation is made
for a new form of existence ; so it is found that the Eustachian
valve is less prominent and the foramen ovale smaller.
PERIODS OF GESTATION.
As a rule, the shorter the period of gestation the more nu-
merous the offspring at a single birth and the greater the num-
ber produced within the lifetime of the animal relatively to its
duration. Thus, on account of the number of young produced
by the rabbit at one birth, the short period of gestation, and the
frequency with which impregnation occurs, there is a much
larger number of progeny, short as is the animal's life usually,
than in the case of the cow, for example, that may bear young
for a much longer period.
The following table gives approximately the duration of the
period of gestation of some of our domestic animals and their
wild allies :
Guinea-pig (cavy) 3 weeks.
Rabbit, squirrel, rat 4 "
Ferret 6 "
Cat S "
Dog, fox 9 "
Lion 4 months.
Sow 4
Sheep, goat 5 "
Bear 7 "
Reindeer 8 "
Cow, buffalo 10 "
128 COMPARATIVE PHYSIOLOGY.
Mare, ass, zebra 11 months.
Camel 12 "
Giraffe 14
Elephant 22 to 25 "
The period of gestation in the human subject is nine months;
in the monkeys and apes somewhat less. The incubation pe-
riod of certain of our domestic birds and related species is about
as follows :
Canary 11 days.
Pigeon 18 "
Hen 21 "
Duck, goose, pea-hen 28 "
Guinea-hen 25
Turkey : 28 '•
It is interesting to note that the smaller varieties of fowls
hatch out sooner than the larger ; and that the period of incu-
bation of all of the above varies with the weather, the steadi-
ness of the incubating bird, the date on which the eggs selected
were laid, etc. With very recent eggs, an attentive sitter, and
warm weather, the incubation period is shortened.
PARTURITION.
All the efforts that have hitherto been made to determine
the exact cause of the result of that series of events which make
up parturition have failed. This has probably been owing to
an attempt at too simple a solution. The foetus lies surrounded
(protected) by fluid contained in the amniotic sac. For its ex-
pulsion there is required, on the one hand, a dilatation of the
uterine opening (os uteri), and, on the other, an expulsive force.
The latter is furnished by the contractions of the uterus itself,
aided by the simultaneous action of the abdominal muscles.
Throughout the greater part of gestation the uterus experiences
somewhat rhythmical contractions, feeble as compared with the
final ones which lead to expulsion of the foetus, but to be re-
garded as of the same character. With the growth and func-
tional development of other organs, the placenta becomes of
less consequence, and a fatty degeneration sets in, most marked
at the periphery, usually where it is thinnest and of least use.
It does not seem rational to believe that the onset of labor is
referable to any one cause, as has been so often taught ; but
rather that it is the final issue to a series of processes long ex-
THE DEVELOPMENT OP THE EMBRYO ITSELF. 129
isting and gradually, though at last rapidly, reaching that
climax which seerus like a vital storm. The law of rhythm
affects the nervous system as others, and upon this depends
the direction and co-ordination of those many activities which
make up parturition. We have seen that. throughout the whole
of foetal life changes in one part are accompanied by correspond-
ing changes in others ; and in the final chapter of this history
it is not to be expected that this connection should be severed,
though it is not at present possible to give the evolution of this
process with any more than a general approach to probable
correctness.
CHANGES IN THE CIRCULATION AFTER BIRTH.
When the new-born mammal takes the first breath, effected
by the harmonious action of the respiratory muscles, excited to
action by stimuli reaching them from the nerve-center (or
centers) which preside over respiration, owing to its being
roused into action by the lack of its accustomed supply of
oxygen, the hitherto solid lungs are expanded ; the pulmonary
vessels are rendered permeable, hence the blood now takes the
path of least resistance along them, as it formerly did through
the ductus arteriosus. The latter, from lack of use, atrophies
in most instances. The blood, returning to the left auricle of
the heart from the lungs in increased volume, so raises the
pressure in this chamber that the stream that formerly flowed
through the foramen ovale from the right auricle is opposed
by a force equal to its own, if not greater, and hence passes by
an easier route into the right ventricle. The fold that tends to
close the foramen ovale grows gradually over the latter, so that
it usually ceases to exist in a few days after birth.
At birth, ligature of the umbilical cord cuts off the placental
circulation ; hence the ductus venosus atrophies and becomes a
mere ligament.
The placenta, being now a foreign body in the uterus, is ex-
pelled, and this organ, by the contractions of its walls, closes
the ruptured and gaping vessels, thus providing against haemor-
rhage.
COITUS.
In all the higher vertebrates congress of the sexes is essential
to bring the male sexual product into contact with the ovum
9
130
COMPARATIVE PHYSIOLOGY.
Erection of the penis results from the conveyance of an ex-
cess of blood to the organ, owing- to dilatation of its arteries,
and the retention of this blood within its caverns.
Fig. 135.— Section of erectile tissue (Cadiat). a, trabecule of connective tissue, with
elastic fibers, and bundles of plain muscular tissue (c); b. venous spaces (Schafer).
The structure of the penis is peculiar, and, for the details of
the anatomy of both the male and female generative organs,
the student is referred to works on this subject ; suffice it to
say that it consists of erectile tissue, the chief characteristic of
which is the opening of the capillaries into cavernous venous
spaces (sinuses) from which the veinlets arise ; with such an
arrangement the circulation must be very slow — the inflow
being greatly in excess of the outflow — apart altogether from
the compressive action of certain muscles connected with the
organ. The manner and duration of the act of copulation in
the domestic animals varies with the structure of the penis, the
animal's nervous excitability, etc. In the stallion the act is of
moderate duration, the penis long, and the glans penis highly
sensitive.
In the bull the penis is of a different shape. During erec-
tion it is believed that the S-shaped curve disappears, and that
the extremity of the organ enters the mouth of the uterus itself.
( !<»l>ulation is of very brief duration.
In the dog the penis is of very peculiar formation. Its an-
THE DEVELOPMENT OF THE EMBRYO ITSELF. 131
terior part contains a bone, while there are two erectile portions
independent of each other. During copulation the largest (pos-
Fig. 136.— Section of parts of three seminiferous tubules of the rat (Schfifer). a, with
the spermatozoa least advanced in development; b, more advanced; c. containing
fully developed spermatozoa. Between the tubules are seen strands of interstitial
cells, with blood-vessels and lymph-spaces.
terior) erectile region is spasmodically (reflexly) grasped by the
sphincter cunni of the female, which is the analogu e of the
bulbo-cavernosus, ischio-cavernosus, and deep transverse mus-
cle of the perinaeum, so that the penis can not be withdrawn
till the erection subsides, an advantage, considering that the
seminal vesicles are wanting in the dog, as well as Cowper's
glands. In the cat tribe there is also an incomplete penial
bone. The free poi*tion of the organ is provided with rigid
papillae capable of erection during copulation.
As previously explained, the spermatozoa originate in the
seminal tubes, from which they find their "way to the seminal
vesicles or receptacles for semen till required to be discharged.
The spermatozoa as they mature are forced on by fresh addi-
tions from behind and by the action of the ciliated cells of the
epididymis, together with the wave-like (peristaltic) action of
the vas deferens. Discharge of semen during coitus is effected
by more vigorous peristaltic action of the vas deferens and the
seminal vesicles, followed by a similar rhythmical action of the
bulbo-cavernosus and ischio-cavernosus muscles, by which the
fluid is forcibly ejaculated.
132
COMPARATIVE PHYSIOLOGY.
PlG. 137. — Generative organs of the mare, isolated and partly opened (Chanvean). 1, 1.
ovaries; 2, Si, Fallopian tubes; 3, pavilion of tube, external face; 4, the same, in-
ner face, showing opening in the middle; 5, ligament of ovary; o, intact horn of
uterus; ',', a born thrown open; 8, body of uterus, upper face; !), broad ligament;
10, cervix, with its mucous folds; 11, cul-de-sac of vagina with its folds of mucous
membrane; 13, urinary meatus and its valve, 14; 15, mucous fold, a vestige of hy-
men; 10, interior of vulva; 17, clitoris; 18, 18, labia of vulva; 19, inferior commis-
sure of vulva.
THE DEVELOPMENT OF THE EMBRYO ITSELF. 133
Semen itself, though composed essentially of spermatozoa,
is mixed with the secretions of the vas deferens, of the seminal
vesicles, of Cowper's glands, and of the prostate. Chemically it
is neutral or alkaline in reaction, highly albuminous, and con-
tains nuclein, lecithin, cholesterin, fats, and salts.
The movements of the male cell, owing to the action of the
tail (cilium), suffice of themselves to convey them to the ovi-
ducts ; but there is little doubt that during or after sexual con-
gress there is in the female, even in the human subject, at least
Fig. 138 —Uterus and ovaries of the sow. semi-diagrammatic (after Dalton). 0, ovary;
II, Fallopian tube; h, horn of the uterus; 5, body of the uterus; v, vagina.
in many cases, a retrograde peristalsis of the uterus and ovi-
ducts which would tend to overcome the results of the activity
of the ciliated cells lining the oviduct. It is known that the
male cell can survive in the female organs of generation for
several days, a fact not difficult to understand, from the method
of nutrition of the female cell (ovum) ; for we may suppose that
both elements are not a little alike, as they are both slightly
modified amoeboid organisms.
Nervous Mechanism — Incidental reference has been made to
the directing influence of the nervous system over the events
of reproduction ; especially their subordination one to another
to bring about the general result. These may now be consid-
ered in greater detail.
Most of the processes in which the nervous system takes
part are of the nature of reflexes, or the result of the automa-
ticity (independent action) of the nerve-centers, increased by
some afferent (ingoing) impressions along a nerve-path. It is
not always possible to estimate the exact share each factor
takes, which must be highly variable. Certain experiments
have assisted in making the matter clear. It has been found
134 COMPARATIVE PHYSIOLOGY.
that if, in a female dog, the spinal cord he divided when the
animal is still a puppy, menstruation and impregnation may
occur. If the same experiment he performed on a male dog,
erection of the penis and ejaculation of semen may he caused
by stimulation of the penis. As the section of the cord has left
the hinder part of the animal's hody severed from the brain,
the creature is, of course, unconscious of anything happening
in all the parts below the section, of whatever nature. If the
nervi erigentes (from the lower part of the spinal cord) be
stimulated, the penis is erected ; and if they be cut this act be-
comes impossible, either reflexly by experiment or otherwise.
Seminal emissions, it is well known, may occur during sleep,
and may be associated, either as result or cause, with voluptu-
ous dreams. Putting all these facts together, it seems reason-
able to conclude that the lower part of the spinal cord contains
the nervous machinery requisite to initiate those influences
("impulses) which, passing along the nerves to the generative
organs, excite and regulate the processes which take place in
them. In these, vascular changes, as we have seen, always
play a prominent part.
Usually we can recognize some afferent influence, either
from the brain (psychical), from the surface — at all events
from without that part of the nervous system (center) which
functions directly in the various sexual processes. It is com-
mon to speak of a number of sexual centers — as the erection
center, the ejaculatory center, etc.— but we much doubt whether
there is such sharp division of physiological labor as these
terms imply, and they are liable to lead to misconception ; ac-
cordingly, in the present state of our knowledge, we prefer to
speak of the sexual center, using even that term in a somewhat
broad sense.
The effects of stimulation of the sexual organs are not con-
fined to the parts themselves, but the ingoing impulses set up
radiating outgoing ones, which affect widely remote areas of
the body, as is evident, especially in the vascular changes:, the
central current of nerve influence breaks up into many streams
as a result of the rapid and extensive rise of the outflowing
current, which breaks over ordinary barriers, and takes paths
which are not properly its own. Bearing this fact in mind,
the chemical composition of semen, so rich in proteid and other
material valuable from a nutritive point of view, and consid-
ering how the sexual appetites may engross the mind, it is not
THE DEVELOPMENT OF THE EMBRYO ITSELF. 135
difficult to understand that nothing so quickly disorganizes the
whole man, physical, mental, and moral, as sexual excesses,
whether by the use of the organs in a natural way, or from
masturbation.
Nature has protected the lower animals by the strong bar-
rier of instinct, so that habitual sexual excess is with them an
impossibility, since the females do not permit of the approaches
of the male except during the rutting period, which occurs
only at stated, comparatively distant periods in most of the
higher mammals. When man keeps his sexual functions in
subjection to his higher nature, they likewise tend to advance
his whole development.
Summary. — Certain changes, commencing with the ripening
of ova, followed by their discharge and conveyance into the
uterus, accompanied by simultaneous and subsequent modifica-
tions of the uterine mucous membrane, constitute, when preg-
nancy occurs, an unbroken chain of biological events, though
usually described separately for the sake of convenience.
When impregnation does not result, there is a retrogression in
the uterus (menstruation) and a return to general quiescence
in all the reproductive organs.
Parturition is to be regarded as the climax of a variety of
rhythmic occurrences which have been gradually gathering
head for a long period. The changes which take place in the
placenta of a degenerative character fit it for being cast off, and
may render this structure to some extent a foreign body before
it and the foetus are finally expelled, so that these changes may
constitute one of a number of exciting causes of the increased
uterine action of parturition. But it is important to regard the
whole of the occurrences of pregnancy as a connected series of
processes co-ordinated by the central nervous system so as to
accomplish one great end. the development of a new individual.
The nutrition of the ovum in its earliest stages is effected by
means in harmony with its nature as an amoeboid organism ;
nutrition by the cells of blood-vessels is similar, while that by
villi may be compared to what takes place through the agency of
similar structures in the alimentary canal of the adult mammal.
The circulation of the foetus puts it on a par physiologically
with the lower vertebrates. Before birth there is a gradual
though somewhat rapid preparation, resulting in changes which
speedily culminate after birth on the establishment of the per-
manent condition of the circulation of extra-uterine life.
136 COMPARATIVE PHYSIOLOGY.
The blood of the foetus (as in the adult) is the great store-
house of nutriment and the common receptacle of all waste
products ; these latter are in the main transferred to the moth-
er's blood indirectly in the placenta; in a similar way nutri-
ment is imported from the mother's blood to that of the foetus.
The placenta takes the place of digestive, respiratory, and excre-
tory organs.
Coitus is essential to bring the male and female elements
together in the higher vertebrates. The erection of the penis is
owing to vascular changes taking place in an organ composed
of erectile tissue ; ejaculation of semen is the result of the
peristaltic action of the various parts of the sexual tract, aided
by rhythmical action of certain striped muscles. The sperma-
tozoa, which are unicellular, flagellated (ciliated) cells, make up
the essential part of semen ; though the latter is complicated
by the addition of the secretions of several glands in connection
with the seminal tract. Though competent by their own move-
ments of reaching the ovum in the oviduct, it is probable that
the uterus and oviduct experience peristaltic actions in a direc-
tion toward the ovary, at least in a number of mammals.
The lower part of the spinal cord is the seat in the higher
mammals of a sexual center or collection of cells that l^eceives
afferent impulses and sends out efferent impulses to the sexual
organs. This, like all the lower centers, is under the control of
the higher centers in the brain, so that its action may be either
initiated or inhibited by the cerebrum.
ORGANIC EVOLUTION RECONSIDERED.
Admitting that the theories of the leading writers on the
subject have advanced us on the way to more complete views of
the mode of origin of the forms of the organic world, it must
still be felt that all theories yet propounded fall short of being
entirely satisfactory. It seems to us unfortunate that the sub-
ject has not received more attention from physiologists, as
without doubt, the final solution must come through that sci-
ence which deals with the properties rather than the forms of
protoplasm ; or, in other words, the fundamental principles
underlying organic evolution are physiological. But, in the
unraveling of a subject of such extreme complexity, all sciences
must probably contribute their quota to make up the truth, as
many rays of different colors compounded form white light.
As with other theories of the inductive sciences, none can be
more than temporary ; there must be constant modification to
meet increasing knowledge. Conscious that any views we our-
selves advance must sooner or later be modified as all others,
even if acceptable now, we venture to lay before the reader the
opinions we have formed upon this subject as the result of con-
siderable thought.
All vital phenomena may be regarded as the resultant of
the action of external conditions and internal tendencies. Amid
the constant change which like involves we recognize two
things : the tendency to retain old modes of behavior, and the
tendency to modification or variatiou. Since those impulses
originally bestowed on matter when it became living, must, in
order to prevail against the forces from without, which tend to
destroy it, have considerable potency, the tendency to modifica-
tion is naturally and necessarily less than to permanence of
form and function.
From these principles it follows that when an Amoeba or
kindred organism divides after a longer or shorter period, it is
138 COMPARATIVE PHYSIOLOGY.
not in reality the same in all respects as when its existence be-
gan, though we ruay be quite unable to detect the changes ; and
when two infusorians conjugate, the one brings to the other
protoplasm different in molecular behavior, of necessity, from
having had different experiences. We attach great importance
to these principles, as they seem to us to lie at the root of the
whole matter. What has been said of these lower but inde-
pendent forms of life applies to the higher. All organisms are
made up of cells or aggregations of cells and their products.
For the present we may disregard the latter. When a muscle-
cell by division gives rise to a new cell, the latter is not identi-
cally the same in every particular as the .parent cell was origi-
nally. It is what its parent has become by virtue of those ex-
periences it has had as a muscle-cell per se, and as a member of
a populous biological community, of the complexities of which
we can scarcely conceive.
Now, as a body at rest may remain so, or may move in a
certain direction according to the forces acting upon it exactly
counterbalance one another, or produce a resultant effect in the
direction in which the body moves, so in the case of heredity,
whether a certain quality in the parent appears in the offspring,
depends on whether this quality is neutralized, augmented, or
otherwise modified by any corresponding quality in the other
parent, or by some opposite quality, taken in connection with
the direct influence of the environment during development.
This assumption explains among other things why acquired
peculiarities (the results of accident, habit, etc.) may or may
not be inherited.
These are not usually inherited because, as is to be expected,
those forces of the organism which have been gathering head
for ages are naturally not easily turned aside. Again, we urge,
heredity must be more pronounced than variation.
The ovum and sperm-cell, like all other cells of the body,
are microcosms representing the whole to a certain extent in
themselves — that is to say, cell A is what it is by reason of what
all the other millions of its fellows in the biological republic
are ; so that it is possible to understand why sexual cells repre-
sent, embody, and repeat the whole biological story, though it
is not yet possible to indicate exactly how they more than others
have this power. This falls under the laws of specialization
and the physiological division of labor ; but along what paths
they have reached this we can not determine.
ORGANIC EVOLUTION RECONSIDERED. 139
Strong- evidence is furnished for the above views by the his-
tory of disease. Scar-tissue, for example, continues to repro-
duce itself as such ; like produces like, though in this instance
the like is in the first instance a departure from the normal.
Gout is well known to be a hereditary disease ; not only so, but
it arises in the offspring at about the same age as in the parent,
which is equivalent to saying that in the rhythmical life of
certain cells a period is reached when they display the behavior,
physiologically, of their parents. Yet gout is a disease that can
be traced to peculiar habits of living and may be eventually
escaped by radical changes in this respect — that is to say, the
behavior of the cells leading to gout can be induced and can be
altered ; gout is hereditary, yet eradicable.
Just as gout may be set up by the formation of certain modes
of action of the cells of the body, so may a mode of behavior,
in the nervous system, for example, become organized or fixed,
become a habit, and so be transmitted to offspring. It will pass
to the descendants or not according to the principles already
noticed. If so fixed in the individual in which it arises as to
predominate over more ancient methods of cell behavior, and
not neutralized by the strength of the normal physiological ac-
tion of the corresponding parts in the other parent, it will reap-
pear. We can never determine whether this is so or not before-
hand ; hence the fact that it is impossible, especially in the case
of man, whose vital processes are so modified by his psychic
life, to predict whether acquired variations shall become heredi-
tary ; hence also the irregulai'ity which characteiizes heredity
in such cases ; they may reappear in offspring or they may not.
In viewing heredity and modification it is impossible to get a
true insight into the matter without taking into the account
both the original natural tendencies of living matter and the
influence of environment. We only know of vital manifes-
tations in some environment ; and, so far as our experience
goes, life is impossible apart from the reaction of surround-
ings. With these general principles to guide us, we shall at-
tempt a brief examination of the leading theories of organic
evolution.
First of all, Spencer seems to be correct in regarding evolu-
tion as universal, and organic evolution but one part of a
whole. No one who looks at the facts presented in every field
of nature can doubt that struggle (opposition, action and reac-
tion) is universal, and that in the organic world the fittest to a
140 COMPARATIVE PHYSIOLOGY.
given environment survives. But Darwin has probably fixed
his attention too closely on this principle and attempted to ex-
plain too much by it, as well as failed to see that there are
other deeper facts underlying- it. Variation, which this author
scarcely attempted to explain, seems to us to be the natural re-
sult of the very conditions under which living things have an
existence. Stable equilibrium is an idea incompatible with our
fundamental conceptions of life. Altered function implies al-
tered molecular action, which sometimes leads to appreciable
structural change. From our conceptions of the nature of liv-
ing matter, it naturally follows that variation should be great-
est, as has been observed, under the greatest alteration in the
surroundings.
We are but very imperfectly acquainted as yet with the
conditions under which life existed in the earlier epochs of the
earth's history. Of late, deep-sea soundings and arctic explo-
rations have brought surprising facts to light, showing that
living matter can exist under a greater variety of conditions
than was previously supposed. Thus it turns out that light is
not an essential for life everywhere. We think these recent
revelations of unexpected facts should make us cautious in
assuming that life always manifested itself under conditions
closely similar to those we know. Variation may at one period
have been more sudden and marked than Darwin supposes;
and there does seem to be rooni for such a conception as the
"extraordinary births'" of Mivart implies; though we would
not have it understood that we think Darwin's view of slow
modification inadequate to produce a new species, we simply
venture to think that he was not justified in insisting so strongly
that this was the only method of Nature; or, to put it more
justly for the great author of the Origin of Species, with the
facts that have accumulated since his time he would scarcely
be warranted in maintaining so rigidly his conviction that
new forms arose almost exclusively by the slow process he has
so ably described.
We must allow a great deal to use and effort, doubtless, and
they explain the origin of variations up to a certain point, but
the solution is only partial. Variations must arise as we have
attempted to explain, and use and disuse are only two of the
factors amid many. Correlated growth, or the changes in one
part induced by changes in another, is a principle which,
though recognized by Darwin, Cope, and others, has not, we
ORGANIC EVOLUTION RECONSIDERED. 141
think, received the attention it deserves. To the mind of the
physiologist, all changes must be correlated -with others.
In what sense has the line that evolution has taken been
predetermined ? In the sense that all things in the universe
are unstable, are undergoing change, leading to new forms and
qualities of such a character that they result in a gradual prog-
ress toward what our minds can not but consider higher mani-
festations of being.
The secondary methods according to which this takes place
constitute the laws of nature, and as wye learn from the prog-
ress of science are very numerous. The unity of nature is a
reality toward which our conceptions are constantly leading
vis. Evolution is a necessity of living matter (indeed, all matter)
as we view it.
THE CHEMICAL CONSTITUTION OF THE
ANIMAL BODY.
One visiting the ruins of a vast and elaborate building-,
which had been entirely pulled to pieces, would get an amount
of information relative to the original structure and uses of
the various parts of the edifice largely in proportion to his fa-
miliarity with architecture and the various trades which make
that art a practical success. The study of the chemistry of
the animal body is illustrated by such a case. Any attempt
to determine the exact chemical composition of living matter
must result in its destruction ; and the amount of information
conveyed by the examination of the chemical ruins, so to speak,
will depend a great deal on the knowledge already possessed of
chemical and vital processes.
It is in all probability true that the nature of any vital pro-
cess is at all events closely bound up with the chemical changes
involved ; but we must not go too far in this direction. We are
not yet prepared to say that life is only the manifestation of
certain chemical and physical processes, meaning thereby such
chemistry and physics as are known to us ; nor are we prepared
to go the length of those who regard life as but the equivalent
of some other force or forces ; as electricity may be considered
as the transformed representative of so much heat and vice versa.
It may be so, but we do not consider that this view is warranted
in the present state of our knowledge.
On the other hand, vital phenomena, when our investiga-
tions are pushed far enough, always seem to be closely asso-
ciated with chemical action : hence the importance to the stu-
dent of physiology of a sound knowledge of chemical princi-
ples. We think the most satisfactory method of studying the
functions of an organ will be found to be that which takes into
consideration the totality of the operations of which it is the
seat, together with its structure and chemical composition ;
CHEMICAL CONSTITUTION OP THE ANIMAL BODY. 143
hence we shall treat chemical details in the chapters devoted to
special physiology, and here give only such an outline as will
bring before the view the chemical composition of the body in
its main outlines ; and even many of these will gather a signifi-
cance, as the study of physiology progresses, that they can not
possibly have at the present.
Fewer than one third of the chemical elements enter into
the composition of the mammalian body ; in fact, the great
bulk of the organism is composed of carbon, hydrogen, nitro-
gen, and oxygen ; sodium, potassium, magnesium, calcium,
sulphur, phosphorus, chlorine, iron, fluorine, silicou, though
occurring in very small quantity, seem to be indispensable to
the living body ; while certain others are evidently only pres-
ent as foreign bodies or impurities to be thrown out sooner
or later. It need scarcely be said that the elements do not
occur as such in the living body, but in combination form-
ing salts, which latter are usually united with albuminous
compounds. As previously mentioned, the various parts which
make up the entire body of an animal are composed of living
matter in very different degrees ; hence we find in such parts
as the bones abundance of salts, relative to the proportion of
proteid matter; a condition demanded by that rigidity without
which an internal skeleton would be useless, a defect well illus-
trated by that disease of the bones known as rickets, in which
the lime-salts are insufficient. It is manifest that there may be
a very great variety of classifications of the compounds found
in the animal body according as we regard it from a chemical,
physical, or physiological point of view, or combine many
aspects in one whole. The latter is, of course, the most correct
and profitable method, and as such is impossible at this stage
of the student's progress ; we shall simply present him with the
following outline, which will be found both simple and com-
prehensive. *
CHEMICAL CONSTITUTION OF THE BODY.
Such food as supplies energy directly must contain carbon
compounds.
Living matter or protoplasm always contains nitrogenous
carbon compounds.
* Taken from the author's Outlines of Lectures on Physiology, W. Drys-
dale & Co., Montreal.
144 COMPARATIVE PHYSIOLOGY.
In consequence, C, H, O, N, are the elements found in great-
est abundance in the body.
The elements S and P are associated with the nitrogenous
carbon compounds ; they also form metallic sulphates and phos-
phates.
CI and F form salts with the alkaline metals Na, K, and the
earthy metals Ca and Mg.
Fe is found in hcemoglobin and its derivatives.
Protoplasm, when submitted to chemical examination, is
killed. It is then found to consist of proteids, fats, carbohy-
drates, salines, and extractives.
It is probable that when living it has a very complex mole-
cule consisting of C, H, O, N, S, and P chiefly.
Proximate Principles.
(a) Nitrogenous. -j Certain crystalliue bodies>
(b) Non-nitrogenous, j ^btfhydrates.
~ T ( Mineral salts.
2. Inorganic, j Water_
Salts. — In general, the salts of sodium are more characteris-
tic of animal tissues and those of potassium of vegetable tissues.
Na CI is more abundant in the fluids of animals ; K and
phosphates more abundant in the tissues.
■ Earthy salts are most abundant in the harder tissues.
The salts are probably not much, if at all, changed in their
passage through the body.
In some cases there is a change from acid to neutral or
alkaline.
The salts are essential to preserve the balance of the nutritive
processes. Their absence leads to disease, e. g., scurvy.
GENERAL CHARACTERISTICS OF PROTEIDS.
They are the chief constituents of most living tissues, includ-
ing blood and lymph.
The molecule consists of a great number of atoms (complex
constitution), and is formed of the elements C, !!,• N, 0, S, and P.
All proteids are amorphous.
All are non-diffusible, the peptones excepted.
They are soluble in strong acids and alkalies, with change of
properties or constitution.
In general, they are coagulated by alcohol, ether, and heating.
CHEMICAL CONSTITUTION OF THE ANIMAL BODY. 145
Coagulated proteids are soluble only in strong acids and
alkalies.
Classification and Distinguishing Characters of Proteids.
1. Native albumins : Serum albumin ; egg albumin ; solu-
ble in water.
2. Derived albumins (albuminates) : Acid and alkali albu-
min ; casein ; soluble in dilute acids and alkalies, insoluble in
water. Not precipitated by boiling.
3. Globulins: Globulin (globin) ; paraglobulin ] myosin;
fibrinogen. Soluble in dilute saline solutions, and precipitated
by stronger saline solutions.
4. Peptones : Soluble in water ; diffusible through anima
membranes; not precipitated by acids, alkalies, or heat. De-
rived from the digestion (peptic, pancreatic) of all proteids.
5. Fibrin : Insoluble in water and dilute saline solutions.
Soluble, but not readily, in strong saline solutions and in dilute
acids and alkalies.
CERTAIN NON-CRYSTALLINE BODIES.
The following bodies are allied to proteids, but are not the
equivalents of the latter in the food.
They are all composed of C, H, N, O. Chondrin, gelatin,
keratin have, in addition, S.
Chondrin : The organic basis of cartilage. Its solutions set
into a firm jelly on cooling.
Gelatin : The organic basis of bone, teeth, tendon, etc. Its
solutions set (glue) on cooling.
Elastin : The basis of elastic tissue. Its solutions do not
set jelly-like (gelatinize).
Mucin : From the secretion of mucous membranes ; precipi-
tated by acetic acid, and insoluble in excess.
Keratin : Derived from hair, nails, epidermis, horn, feathers.
Highly insoluble.
Nuclein: Derived from the nuclei of cells. Not digested
by pepsin ; contains P but no S.
THE FATS.
The fats are hydrocarbons ; are less oxidized than the carbo-
hydrates ; are inflammable ; possess latent energy in a high
degree.
Chemically, the neutral fats are glycerides or ethers of the
10
146 COMPARATIVE PHYSIOLOGY.
fatty acids, i. e., the acid radicles of the fatty acids of the oleic
and acetic series replace the exchangeable atoms of H in the
triatomic alcohol glycerine, e. g. :
Glycerine. Palmitic acid. Glycerine tripalmitate or palmitin.
i OH HO.OC.Ci6H3l l O.CO.C16H31
C3H6 \ OH + HO.OC.C15H31 = C3H5 \ O.CO.C15H31 + 3H20
( OH HO.OC.Ci&H31 ( O.CO.Ci5H31
A soap is formed by the action of caustic alkalies on fats, e. g. :
Tripalmitin. Potassium palmitate.
(c,?h% \ °> + 3 <K0H> = 3 j (g'*Hl,0)o f +C,I; f o,
The soap may be decomposed by a strong acid into a fatty
acid and a salt, e. g. :
ClHslCCK + HC1 = Ci6H,i.COsH + KC1.
Potassium palmitate. Palmitic acid.
The fats are insoluble in water, but soluble in hot alcohol,
ether, chloroform, etc.
The alkaline soaps are soluble in water.
Most animal fats are mixtures of several kinds in varying
proportions ; hence the melting-point for the fat of each species
of animal is different.
PECULIAR FATS.
Lecithin, Protagon, Cerebrin :
They consist of C, H, N, O, and the first two of P in addi-
tion.
They occur in the nervous tissues.
CARBOHYDRATES.
General formula, Cm (H20)„.
1. The Sugars : Dextrose, or grape-sugar, C0H12O6 readily
undergoes alcoholic fermentation ; less readily lactic fermen-
tation.
Lactose, milk-sugar, Ci2HMOn ; susceptible of the lactic acid
fermentation.
Inosit, or muscle-sugar, C0H,nO0 ; capable of the lactic fer-
mentation.
Maltose, C12HM0„, capable of the alcoholic fermentation.
The chief sugar of the digestive process.
All the above are much less sweet and soluble than ordinary
cane-sugar.
CHEMICAL CONSTITUTION OF THE ANIMAL BODY. 147
2. The Starches : Glycogen, CcH10O5, convertible into dex-
trose. Occurs abundantly in many foetal tissues and in the
liver, especially of the adult animal.
Dextrin, C6Hjo06, convertible into dextrose. Soluble in
water ; intermediate between starch and dextrose ; a product
of digestion.
Pathological : Grape-sugar occurs in the urine in diabetes
mellitus.
Certain substances formed within the body may be regarded
as chiefly waste-products, the result of metabolism or tissue-
changes.
They are divisible into nitrogenous metabolites and non-
nitrogenous metabolites.
Nitrogenous Metabolites.
1. Urea, uric acid and compounds, kreatinin, xanthin, hypo-
xanthin (sarkin), hippuric acid, all occuring in urine.
2. Leucin, tyrosin, taurocholic, and glycocholic acids, which
occur in the digestive tract.
3. Kreatin, constantly found in muscle, and a few others of
less constant occurrence.
The above consists of C, H, N, O. Taurocholic acid contains
also S.
The molecule in most instances is complex.
Non-Nitrogenous Metabolites.
These occur in small quantity, and some of them are secreted
in an altered form.
They included lactic and sarcolactic acid, oxalic acid, succinic
acid, etc.
PHYSIOLOGICAL RESEAKCH AND
PHYSIOLOGICAL SEASONING.
We propose in this chapter to examine into the methods
employed in physiologcial investigation and teaching, and the
character of conclusions arrived at hy physiologists as depend-
ent on a certain method of reasoning.
The first step toward a legitimate conclusion in any one of
the inductive sciences to which physiology belongs is the col-
lection of facts which are to constitute the foundation on
which the inference is to be based. If there be any error in
these, a correct conclusion can not be drawn by any reliable
logical process. On the other hand, facts may abound in thou-
sands and yet the correct conclusion never be reached, because
the method of interpretation is faulty, which is equivalent to
saying that the process of inference is either incomplete or in-
correct. The conclusions of the ancients in regard to nature
were usually faulty from errors in both these directions ; they
neither had the requisite facts, nor did they correctly interpret
those with which they were conversant.
Let us first examine into the methods employed by modern
physiologists, and determine in how far they are reliable. First,
there is the method of direct observation, in which no appara-
tus whatever or only the simplest kind is employed ; thus, the
student may count his own respirations, feel his own heart-
beats, count his pulse, and do a very great deal more that will
be pointed out hereafter; or he may examine in like manner an-
other fellow-being or one of the lower animals. This method
is simple, easy of application, and is that usually employed by
the physician even at the present day, especially in private
practice. The value of the results obviously depends on the
reliability of the observer in two respects : First, as to the ac-
curacy, extent, and delicacy of his perceptions ; and, secondly,
on the inferences based on these sense-observations. Much
PHYSIOLOGICAL RESEARCH AND REASONING. 149
must depend on practice — that is to say, the education of the
senses. The hand may become a most delicate instrument of
observation ; the eye may learn to see what it once could not ;
the ear to detect and discriminate what is quite beyond the
uncultured hearing of the many. But it is one of the most
convincing evidences of man's superiority that in every field of
observation he has risen above the lower animals, some of which
by their unaided senses naturally excel him. So in this science,
instruments have opened up mines of facts that must have other-
wise remained hidden ; they have, as it were, provided man
with additional senses, so much have the natural powers of
those he already possessed been sharpened.
But the chief value of the results reached by instruments *
consists in the fact that the movements of the living tissues can
be registered ; i. e., the great characteristic of modern physiol-
ogy is the extensive employment of the graphic method, which
has been most largely developed by the distinguished French
experimenter Marey. Usually the movements of the point of
lever are impressed on a smoked surface, either of glazed
paper or glass, and rendered permanent by a coating of some
material applied in solution and drying quickly, as shellac in
alcohol. The surface on which the tracing is written may be
stationary, though this is rarely the case, as the object is to get
a succession of records for comparison ; hence the most used
form of writing surface is a cylinder which may be raised or
lowered, and which is moved around regularly by some sort of
clock-work. It follows that the lever point, which is moved by
the physiological effect, describes curves of varying complexity.
That tracings of this or any other character should be of any
value for the purposes of physiology, they must be susceptible
of relative measurement both for time and space. This can be
accomplished only when there is a known base-line or abscissa
from which the lever begins its rise, and a time record which is
usually in seconds or portions of a second. The first is easily
obtained by simply allowing the lever to write a straight line
before the physiological effect proper is recorded. Time inter-
vals are usually indicated by the interruptions of an electric
current, or by the vibrations of a tuning-fork, a pen or writer
of some kind being in each instance attached to the apparatus
so as to record its movements.
* Illustrated in the sections on muscle physiology and others.
150 COMPARATIVE PHYSIOLOGY.
As levers, in proportion to their length, exaggerate all the
movements imparted to them, a constant process of correction
must be carried on in the mind in reading the records of the
graphic method, as in interpreting the field of view presented
by the microscope.
The student is epecially warned to carry on this process,
otherwise highly distorted views of the reality will become
fixed in his own mind ; and certainly a condition of ignorance
is to be preferred to such false knowledge as this may become.
But it is likewise apparent that movements that would without
such mechanism be quite unrecognized may be rendered visible
and utilized for inference. There is another source of possible
misconception in the use of the graphic method. The lever is
sometimes used to record the movements of a column of fluid
(manometer, Fig. 197), as water or mercury, the inertia of which
is considerable, so that the record is not that of the lever as
affected by the physiological (tissue) movement, but that move-
ment conveyed through a fluid of the kind indicated. Again,
all points, however delicate, write with some friction, and the
question always arises, In how far is that friction sufficient to
be a source of inaccuracy in the record ? When organs are di-
rectly connected with levers or apparatus in mechanical rela-
tion with them, one must be sure that the natural action of the
organ under investigation is in no way modified by this con-
nection.
From these remarks it will be obvious that in the graphic
method physiologists possess a means of investigation at once
valuable and liable to mislead. Already electricity has been
extensively used in the researches of physiologists, and it is to
this and the employment of photography that we look in the
near future for methods that are less open to the objections we
have noticed.
However important the methods of physiology, the results
are vastly more so. We next notice, then, the progress from
methods and observations to inferences, which we shall en-
deavor to make clear by certain cases of a hypothetical charac-
ter. Proceeding from the brain and entering the substance of
the heart, there is in vertebrates a nerve known as the vagus.
Suppose that, on stimulating this nerve by electricity in a rab-
bit, the heart ceases to beat, what is the legitimate inference ?
Apparently that the effect has been due to the action of the
nerve on the heart, an action excited by the use of electricity.
PHYSIOLOGICAL RESEARCH AND REASONING. 151
This does not, however, according to the principles of a rigid
logic, follow. The heart may have ceased beating from some
cause wholly unconnected with this experiment, or from the
electric current escaping along the nerve and affecting some
nervous mechanism within the heart, which is not a part of the
vagus nerve ; or it may have been due to the action of the cur-
rent on the muscular tissue of the heart directly, or in some other
way. But suppose that invariably, whenever this experiment
is repeated, the one result (arrest of the beat) follows, then it is
clear that the vagus nerve is in some way a factor in the causa-
tion. Now, if it could be ascertained that certain branches of
the nerve were distributed to the heart-muscle directly, and that
stimulation of these gave rise to arrest of the cardiac pulsation,
then would it be highly probable, though not certain, that there
was in the first instance no intermediate mechanism ; while
this inference would become still more probable if in hearts
totally without any such nervous apparatus whatever, such a
result followed on stimulation of the vagus. Suppose, further,
that the application of some drug or poison to the heart pro-
vided with special nervous elements besides the vagus termi-
nals prevented the effect before noticed on stimulating the
vagus, while a like result followed under similar circumstances
in those forms of heart unprovided with such nervous struct-
ures, there would be additional evidence in favor of the view
that the result we are considering was due solely to some action
of the vagus nerve ; while, if arrest of the heart followed in the
first case but not in the second, and this result were invariable,
there would be roused the suspicion that the action of the
vagus was not direct, but through the nervous structures with-
in the heart other than vagus endings. And if, again, there were
a portion of the rabbit's heart to which there were distributed
this intrinsic nervous supply, which on stimulation directly
was arrested in its pulsation, it would be still more probable
that the effect in the first instance we have considered was clue
to these structures, and only indirectly to the vagus. But be it
observed, in all these cases there is only probability. The con-
clusions of physiology never rise above probability, though this
may be so strong as to be practically equal in value to absolute
certainty. Would it be correct, from any or all the experi-
ments we have supposed to have been made, to assert that the
vagus was the arresting (inhibitory) nerve of the heart ? All
hearts thus far examined have much in common in structure
152 COMPARATIVE PHYSIOLOGY.
and function, and in so far is the above generalization probable.
Such a statement would, however, be far from that degree of
probability which is possible, and should therefore not be ac-
cepted till more evidence has been gathered. The mere resem-
blance in form and general function does not suffice to meet the
demands of a critical logic. Such a statement as the above would
not necessarily apply to the hearts of all vertebrates or even all
rabbits, if the experiments had been conducted on one animal
alone, for the result might be owing to a mere idiosyncrasy of
the rabbit under observation. The further we depart from the
group of animals to which the creature under experiment be-
longs, the less is the probability that our generalizations for
the one class will apply to another. It will, therefore, be seen
that wide generalizations can not be made with that amount of
certainty which is attainable until experiments shall have be-
come very numerous and widely extended. A really broad and
sound j)hysiology can only be constructed when this science
has become much more comparative — that is, extended to many
more groups and sub-groups of animals than at present.
We have incidently alluded throughout the work to the
teaching of disease. " Disease " is but a name for disordered
function. One viewing a piece of machinery for the first time
in improper action might draw conclusions with comparative
safety, provided he had a knowledge of the correct action of
similar machines. Our experience gives us a certain knowl-
edge of the functions of our own bodies. By ordinary observa-
tion and by experiment on other animals we get additional
data, which, taken with the disordered action resulting from
gross or molecular injury (disease), gives a basis for certain
conclusions as to the normal functions of the human body or
those of lower animals. This information is especially valu-
able in the case of man, since he can report with a fair de-
gree of reliability, in most diseased conditions, his own sensa-
tions.
It is hoped that this brief treatment of the methods and
logic of physiology will suffice for the present. Throughout
the work they will be illustrated in every chapter, though not
always with distinct references to the nature of the intellectual
process followed.
Summary. — There are two methods of physiological observa-
tion, the direct and the indirect. The first is the simplest, and
is valuable in proportion to the accuracy and delicacy and
PHYSIOLOGICAL RESEARCH AND REASONING. 153
range of the observer ; the latter implies the use of apparatus,
and is more complex, more extended, more delicate, and precise.
It is usually employed with the graphic method, which has the
advantage of recording and thus preserving movements which
correspond with more or less exactness to the movements of
tissues or organs. It is valuable, but liable to errors in record-
ing and in interpretation.
The logic of physiology is that of the inductive sciences. It
proceeds from the special to the general. The conclusions of
physiology never pass beyond extreme probability, which, in
some cases, is practically equal to certainty. It is especially
important not to make generalizations that are too wide.
THE BLOOD.
It is a matter of common observation that the loss of the
whole, or a very large part, of the blood of the body entails
death ; while an abundant haemorrhage, or blood-disease in any
of its forms, causes great general weakness.
The student of embryology is led to inquire as to the neces-
sity for the very early appearance and the rapid development
of the blood- vascular system so prominent in all vertebrates.
An examination of the means of transit of the blood, as
already intimated, reveals a complicated system of tubes dis-
tributed to every organ and tissue of the body. These facts
would lead one to suppose that the blood must have a tran-
scendent importance in the economy, and such, upon the most
minute investigation, proves to be the case. The blood has
been aptly compared to an internal world for the tissues, an-
swering to the external world for the organism as a whole.
This fluid is the great storehouse containing all that the most
exacting cell can demand ; and, further, is the temporary re-
ceptacle of all the waste that the most busy cell requires to dis-
charge. Should such a life-stream cease to flow, the whole vital
machinery must stop — death must ensue.
Comparative. — It will prove more scientific and generally
satisfactory to regard the blood as a tissue having a fluid and
flowing matrix, in which flow cellular elements or corpuscles —
a view of the subject that is less startling when it is remem-
bered that the greater part of the protoplasm which makes up
the other tissues of the body is of a semifluid consistence. In
all animals possessing blood, the matrix is a clear, usually more
or less colored fluid. Among invertebrates the color may be
pronounced : thus, in cephalopods and some crustaceans it is
blue, but in most groups of animals and all vertebrates the
matrix is either colorless or more commonly of some slight
tinge of yellow. Invertebrates with few exceptions possess
THE BLOOD.
155
only colorless corpuscles, but all vertebrates have colored cells
which invariably outnumber the other variety, and display
forms and sizes
which are sufficient-
ly constant to be
characteristic. In all
groups below mam-
mals the colored cor-
puscles are oval,
mostly biconvex,
and nucleated dur-
ing all periods of the
animal's existence ;
in mammals they are
circular biconcave
disks (except in the
camel tribe, the cor-
puscles of which are
oval), and in post-
embryonic life with-
out a nucleus ; nor
do they possess a
cell-wall. The red
cells vary in size in different groups and sub-groups of animals,
being smaller the higher the place the animal occupies, as a
general rule ; thus, they
are very large in verte-
brates below mammals,
in some cases being al-
most visible to the un-
aided eye, while in the
whole class of mam-
mals they are very mi-
nute ; their numbers
also in this group are
vastly greater than in
others lower in the
scale.
The average size in
man is ^5 inch ('0077
mm.) and the nnmber
in a cubic millimetre
Fig. 139. — Leucocytes of human blood, showing amoe-
boid movements (Landois). These movements are
not normally in the blood-vessels so marked as pic-
tured here, so that the figure represents an extreme
case.
Fia. 140.-
-Photograph of colored corpuscles of frog.
1 x"370. (After Flint.).
156
COMPARATIVE PHYSIOLOGY.
of the blood about 5,000,000 for the male and 500,000 less for
the female, which would furnish about 250,000,000,000 in a
pound of blood. It will be understood that averages only are
spoken of, as all kinds of variations occur, some of which will
be referred to later, and their significance explained. The size
of the corpuscles in the domestic animals is variable — a matter
of importance when transfusion of blood is under consideration.
Under the microscope the blood of vertebrates is seen to owe
its color to the cells chiefly, and, so far as the red goes, almost
wholly. Corpuscles
when seen singly are
never of the deep red,
however, of the blood
as a whole, but rather
a yellowish red, the
tinge varying some-
what with the class of
animals from which
the specimen has been
taken.
Certain other mor-
phological elements
found in mammalian
blood deserve brief
mention, though their
significance is as yet
a matter of much dis-
pute.
1. The blood-plates
(plaques, hcematoblasts, third element), very small, colorless,
biconcave disks, which are deposited in great numbers on any
thread or similar foreign body introduced into the circulation,
and rapidly break up when blood is shed.
2. On a slide of blood that has been prepared for some little
time, aggregations of very minute granules (elementary gran-
ules) may be seen. These are supposed to represent the disin-
tegrating protoplasm of the corpuscles.
The pale or colorless corpuscles are very few in number in
mammals compared with the red, there being on the average
only about 1 in 400 to GOO, though they become much more
numerous after a meal. They are granular in appearance, and
possess one or more nuclei, which are not, however, readily
Fig. 141.— Corpuscles from human subject (Pnnke).
A few colorless corpuscles are seen among the col-
ored disks, which are many of them arranged in
rouleaux.
THE BLOOD.
157
seen in all cases without the use of reagents. They are charac-
terized by greater size, a globular form, the lack of pigment,
©
s
Fia. 142.— Blood-plaques and their derivatives (Landois, after Bizzozero and Laker).
1, red blood-corpuscles on the flat; 2, from the side; 3, unchanged blood-plaques;
4, lymph-corpuscle surrounded with blood-plaques; 5, blood-plaques variously
altered; 6, lymph-corpuscle with two masses of fused blood-plaques and threads
of fibrin; 7, group of blood-plaques fused or run together; 8, similar small mass
of partially dissolved blood-plaques with fibrils of fibrin.
and the tendency to amoeboid movements, which latter may be
exaggerated in disordered conditions of the blood, or when the
blood is withdrawn and observed under artificial conditions.
It will be understood that these cells (leucocytes) are not con-
fined to the blood, but abound in lymph and other fluids.
They are the representatives of the primitive cells of the em-
bryo, as is shown by their tendency (like ova) to throw out
processes, develop into higher forms, etc. In behavior they
strongly suggest Amoeba and kindred forms.
We may, then, say that in all invertebrates the blood, when
it exists, consists of a plasma (liquor sanguinis), in which float
the cellular elements which are colorless ; and that in verte-
brates in addition there are colored cells which are always nu-
cleated at some period of their existence. The colorless cells
are globular masses of protoplasm, containing one or more
nuclei, and with the general character of amoeboid organisms.
158
COMPARATIVE PHYSIOLOGY.
bx:
The History of the Blood-Cells.
We have already seen that the blood and the vessels in
which it flows have a common origin in the mesoblastic cells of
the embryo chick ; the same applies to mammals and lower
groups. The main facts may be grouped under two head-
ings : 1. Development of tbe blood-corpuscles during embry-
onic life. 2. Development of the corpuscles in post-embryonic
life. The origin and fate of the corpuscles, especially of the
colored variety, have been the subject of much discussion.
The best established
facts are stated in the
summary below, while
they are illustrated by
the accompanying fig-
ures.
The colorless cells
of the blood first arise
as migrated uu differen-
tiated remnants of the
eai'ly embryonic cell
colonies. That they re-
main such is seen by
their physiological be-
havior, to be considered
a little later. Afterward
they are chiefly pro-
duced from a peculiar
form of connective tis-
sue known as leucocy-
tenic, and which is
gathered into organs
(lymphatic glands), the
chief function of which is to produce these cells, though this
tissue is rather widely distributed in the mammalian body in
other forms than these.
Summary. — The student may, with considerable certainty,
consider the colorless corpuscle of the blood as the most primi-
tive; the red, derived either from the white or some form of
more specialized cell ; the nucleated, as the earlier and more
youthful form of the colored corpuscle, which may in some
groups of vertebrates be replaced by a more specialized (or de-
Fig. 143.— Surface view from below of a small por-
tion of posterior end of pellucid area of a cliick
of thirty-six hours, 1 x 400 (Foster and Balfour).
b. c, blood-corpuscles ; a, nuclei, which subse-
quently become nuclei of cells forming walls of
blood-vessels ; p. pr, protoplasmic processes,
containing nuclei with large nucleoli, n.
THE BLOOD.
159
graded ?) non-nucleated form mostly derived directly from the
former ; that in the first instance the blood-vessels and blood
© 0 © |
Fig. 144.
Fig. 145.
Fig. 146.
a
Fig. 147.
Fig. 148.
Fig. 144.— Cell elements of red marrow, a, large granular marrow cells; b, smaller,
more vesicular cells; c. free nuclei, or small lymphoid cells, some of which may
be even surrounded with a delicate rim of protoplasm; d, nucleated red corpuscles
of the bone marrow.
Fig. 145. — Nucleated red cells of marrow, illustrating mode of development into the
ordinary non-nucleated red corpuscles, a. common forms of the colored nucleated
cells of red marrow; b, 1. 2, 3. gradual disappearance of the nucleus; c, large non-
nucleated red corpuscle resembling 2 and 3 of b in all respects save in the absence
of any trace of nucleus.
Fig. 146.— Nucleated red corpuscles, illustrating the migration of the nucleus from the
cell, a process not unfrequently seen in the red marrow.
Fig. 147.— Blood of human embryo of four months. «, 1, 2, 3. 4, nucleated red corpus-
cles. In 4 the same granular disintegrated appearance of the nucleus as is noted
in marrow cells, b. 1. microcyte; 2, megalocyte; 3, ordinary red corpuscle.
Fig. 148.— From spleen. 1, blood-plaques, colorless and varying a little in size; 2, two
microcytcs of a deep-red color; 3. two ordinary red corpuscles; 4, a solid, translu-
cent, lymphoid cell or free nucleus. (Figs. 144-148 after Osier.)
arise simultaneously in the mesohlastic embryonic tissue ; that
such an organ may exist after birth, either normally in some
mammals or under unusual functional need ; that the red mar-
row is the chief birthplace of colored cells in adult life ; that
160 COMPARATIVE PHYSIOLOGY.
the spleen, liver, lymphatic glands, and other tissues of similar
structure contribute in a less degree to the development of the
red corpuscles ; and that the last mentioned organs are the chief
producers of the colorless amoeboid blood-cells.
Finally, it is well to remember that Nature's resources in
this, as in many other cases, are numerous, and that her mode
of procedure is not invariable ; and that, if one road to an end
is blocked, another is taken.
The Decline and Death of the Blood-Cells.— The blood cor-
puscles, like other cells, have a limited duration, with the usual
chapters in a biological history of rise, maturity, and decay.
There is reason to believe that the red cells do not live longer
than a few weeks at most. The red cells, in various degrees of
disorganization, have been seen within the white cells {phagocy-
tes), and the related cells of the spleen, liver, bone-marrow, etc.
In fact, these cells, by virtue of retained ancestral {amoeboid)
qualities, have devoured the weakened, dying red cells. It seems
to be a case of survival of the fittest. It is further known that
abundance of pigment containing iron is found in both spleen
and liver ; and there seems to be no good reason for doubting
that the various pigments of the secretions of the body {urine,
bile, etc.) are derived from the universal pigment of the blood.
These coloring matters, then, are to be regarded as the excreta
in the first instance of cells behaving like amceboids, and later
as the elaborations of certain others in the kidney and else-
where, the special function of which is to get rid of waste prod-
ucts. The birth-rate and the death-rate of the blood-cells must
be in close relation to each other in health ; and some of the
gravest disturbances arise from decided changes in the normal
proportions of the cells {anaimia, leucocyt hernia).
Both the red and white corpuscles show, like all other cells
of the organism, alterations corresponding to changes in the
surrounding conditions. The blood may be withdrawn and its
cells more readily observed than those of most tissues ; so that
the study of the influence of temperature, feeding of the leuco-
cytes, and the action of reagents in both classes of cells is both
of practical importance and theoretic interest, and will well re-
pay the student for the outlay in time and labor, if attention is
directed chiefly to the results and the lessons they convey, and
not, as too commonly happens, principally to the methods of
manipulation.
The Chemical Composition of the Blood,— Blood has a decided
THE BLOOD. 161
but faint alkaline reaction, owing1 chiefly to the presence of
sodium salts, a saline taste, and a faint odor characteristic of
the animal group to which it belongs, owing probably to volatile
fatty acids. The specific gravity of human blood varies between
1045 and 1075, with a mean of 1055 ; the specific gravity of the
corpuscles being about 1105 and of the plasma 1027. This dif-
ference explains the sinking of the corpuscles in blood with-
drawn from the vessels and kept quiet. Much the same diffi-
culties are encountered in attempts at the exact determination
of the chemical composition of the blood, as in the case of other
living tissues. Plasma alters its physical and its chemical com-
position, to what extent is not exactly known, when removed
from the body.
Composition of Serum.— The fluid remaining after coagula-
tion of the blood can, of course, be examined chemically with
considerable thoroughness and confidence.
By far the greater part of serum consists of water ; thus, it
has been estimated that of 100 parts the following statement will
represent fairly well the proportional composition :
Water 90 parts ;
Proteids 8 to 9 "
Salines, fats, and extractives (small in
quantity and not readily obtained
free) 1 to 2 parts.
The proteids are made up of two substances which can be
distinguished by solubility, temperature at which coagulation
occurs, etc., known as paraglobulin and serum-albumen, and
which may exist in equal amount.
It is not possible, of course, to say whether these substances
exist as such in the living blood-plasma or not.
The fats are very variable in quantity iu serum, depend-
ing on a corresponding variability in the plasma, in which
they would be naturally found in greatest abundance after a
meal. They exist as neutral stearin, palmitin, olein, and as
soaps.
The principal extractives found are urea, creatin and allied
bodies, sugar, and lactic acid. Serum in most animals contains
more of sodium salts than the corpuscles, while the latter in
man and some other mammals contain a preponderating quan-
tity of potassium compounds.
The principal salts of serum are sodium chloride, sodium bi-
carbonate, sodium sulphate and phosphate ; in smaller quantity,
11
162 COMPARATIVE PHYSIOLOGY.
also phosphate of calcium and magnesium, with rather more
of potassium chloride.
It is highly prohahle that this proportion also represents
moderately well the composition of plasma, which is, of course,
from a physiological point of view, the important matter.
The Composition of the Corpuscles.— Taken together, the dif
ferent forms of blood-cells make up from one third to nearly
one half the weight of the blood, and of this the red corpuscles
may be considered as constituting nearly the whole.
The colorless cells are known to contain fats and glycogen,
which, with salts, we may believe exist in the living cells, and,
in addition to the proteids, into which protoplasm resolves it-
self upon the disorganization that constitutes its dying, lecithin,
protagon, and other extractives.
The prominent chemical fact connected with the red corpus-
cles is their being composed in great part of a peculiar colored
proteid compound containing iron.
This will be fully considered later: but, in the mean time
we may state that the haemoglobin is itself infiltrated into the
meshes or framework (stroma) of the corpuscle, which latter
seems to be composed of a member of the globulin class, so well
characterized by solubility in weak saline solutions.
The following tabular statement represents the relative pro-
portions in 100 parts of the dried organic matter of the red cor-
puscles :
Haemoglobin 90 54
Proteids 8-67
Lecithin. 0"54
Cholesterin 0-25
100-00
The quantity of salts is very small, less than one per cent
(inorganic).
So much for the results of our analyses ; but when we con-
sider the part the blood plays in the economy of the body, it
must appear that, since the life-work of every cell expresses it-
self through this fluid, both as to what it removes and what it
adds, the blood can not for any two successive moments be of
precisely the same composition ; yet the departures from a nor-
mal standard must be kept within very narrow limits, other-
wise derangement or possibly death results. We think that,
before we have concluded the study of the various organs of
THE BLOOD. 163
the body, it will appear to the student, as it does to the writer,
that it is highly probable that there are great numbers of com-
pounds hi the blood, either of a character unknown as yet to
our chemistry, or in such small quantity that they elude detec-
tion by our methods ; and we may add that we believe the same
holds for all the fluids of the body. The complexity of vital
processes is great beyond our comprehension.
It must be especially borne in mind that all the pabulum
for every cell, however varied its needs, can be derived from
the blood alone ; or, as we shall show presently, strictly speak-
ing from the lymph, a sort of middle-man between the blood
and the tissues.
The Quantity and the Distribution of the Blood.— The rela-
tive quantities of blood in different parts of the body have been
estimated to be as follows:
Liver one fourth.
Skeletal muscles " "
Heart, lungs, large arteries, and veins. " "
Other structures ... " "
The significance of this distribution will appear later.
The Coagulation of the Blood.— When blood is removed
from its accustomed channels, it undergoes a marked chemical
and physical change, termed clotting or coagulation. In the
case of most vertebrates, almost as soon as the blood leaves the
vessels it begins to thicken, and gradually acquh'es a consistence
that may be compared to that of jelly, so that it can no longer
be poured from the containing vessel. Though some have rec-
ognized different stages as distinct, and named them, we think
that an unprejudiced observer might fail to see that there were
any well-marked appearances occurring invariably at the same
moment, or with resting stages in the process, as with the devel-
opment of ova.
After coagulation has reduced the blood to a condition in
which it is no longer diffluent, minute drops of a thin fluid
gradually show themselves, exuding from the main mass, faintly
colored, but never red, if the vessel in which the clot has
formed has been kept quiet so that the red corpuscles have not
been disturbed ; and later it may be noticed that the main mass
is beginning to sink in the center {cupping) ; and in the blood
of certain animals, as the horse, which clots slowly, the upper
part of the coagulum (cr assume ntum) appears of a lighter
color, owing, as microscopic examination shows, to the relative
164 COMPARATIVE PHYSIOLOGY.
fewness of red corpuscles. This is the huffy -coat, or, as it oc-
curs in inflammatory conditions of the hlood, was termed by
older writers, the crusta plilogistica. It is to be distinguished
from the lighter red of certain parts of a clot, often the result of
greater exposure to the air and more complete oxidation in con-
sequence. The white blood-cells, being lighter than the red, are
also more abundant in the upper part of the clot (buffy-coat).
If the coagulation of a drop of blood withdrawn from one's
own finger be watched under the microscope, the red corpuscles
may be seen to run into heaps, like rows of coins lying against
each other (rouleaux, Fig. 141), and threads of the greatest
fineness are observed to radiate throughout the mass, gradually
increasing in number, and, at last, including the whole in a
meshwork which slowly contracts. It is the formation of this
fibrin which is the essential factor in clotting; the inclusion of
the blood-cells and the extrusion of the serum naturally result-
ing from its formation and contraction.
The great mass of every clot consists, however, of corpuscles ;
the quantity of fibrin, though variable, not amounting to more
usually than about '2 per cent in mammals. Tbe formation of
the clot does not occupy more than a few minutes (two to seven)
in most mammals, including man, but its contraction lasts a
very considerable time, so that serum may continue to exude
from the clot for hours. It is thus seen that, instead of the
plasma and corpuscles of the blood a%it exists within the living
body, coagulation has resulted in the formation of two new
products — serum and fibrin — differing botli physically and
chemically. These facts may be put in tabular form thus:
Blood as it flows ) Liquor sanguinis (plasma).
in tbe vessels. ( Corpuscles.
Blood after co- \ Coagulum | £^cles<
agulation. j Serum.
As fibrin may be seen to arise in the form of threads, under
the microscope, in coagulating blood, and since no trace of it in
any form has been detected in the plasma, and the process can
be accounted for otherwise, it seems unjustifiable to assume that
fibrivi exists preformed in the blood, or arises in any way prior
to actual coagulation.
Fibrin belongs to the class of bodies known as proteids, and
can be distinguished from the other subdivisions of this group
of substances by certain chemical as well as physical character-
THE BLOOD. 165
istics. It is insoluble in water and in solutions of sodium chlo-
ride; insoluble in hydrochloric acid, though it swells in this
menstruum.
It may be whipped out from the freshly shed blood by a
bundle of twigs, wires, or other similar arrangement presenting
a considerable extent of surface ; and when washed free from
red blood-cells presents itself as a white, stringy, tough sub-
stance, admirably adapted to retain anything entangled in its
meshes. If fibrin does not exist in the plasma, or does not arise
directly as such in the clot, it must have some antecedents al-
ready existing as its immediate factors in the plasma, either
before or after it is shed.
The principal theories of coagulation are these : 1. Coagu-
lation results from the action of a fibrin-ferment on fibrinogen
and paraglobulin. 2. Coagulation results from the action of
a fibrin-ferment on fibrinogen alone. Fibrinogen and para-
globulin (see sections on " The Chemistry of the Animal Body")
are proteids originating from the plasma, during clotting in all
probability. Fibrin-ferment loses its properties on boiling, and
a very small quantity suffices in most cases to induce the result.
For these and other reasons this agent has been classed among
bodies known as unorganized ferments, which are distinguished
by the following properties :
They exert their influence only under well-defined circum-
stances, among which is' a certain narrow range of tempera-
ture, about blood-heat being most favorable for their action.
They do not seem to enter themselves into the resulting prod-
uct, but act from without, as it were (catalytic action), hence a
very small quantity suffices to effect the result. In all cases
they are destroyed by boiling, though they bear exposure for
a limited period to a freezing temperature.
From observations, microscopic and other, it has been con-
cluded that the corpuscles play an important part in coagula-
tion by furnishing the fibrin-ferment ; but the greatest diver-
sity of opinion prevails as to which one of the morphological
elements of the blood furnishes the ferment, for each one of
them has been advocated as the exclusive source of this fer-
ment bjr different observers.
We do not favor the current theories of the coagulation of
the blood. We would explain the whole matter somewhat thus:
What the blood is in chemical composition and other properties
from moment to moment is the result of the complicated inter-
166 COMPARATIVE PHYSIOLOGY.
action of all the various cells and tissues of the body. Any one
of these, departing from its normal behavior, at once affects the
blood ; but health implies a constant effort toward a certain
equilibrium, never actually reached but always being striven
after by the whole organism. The blood can no more maintain
its vital equilibrium, or exist as a living tissue out of its usual
environment, than any other tissue. But the exact circum-
stances under which it may become disorganized, or die, are
legion ; hence, it is not likely that the blood always clots in
the same way in all groups of animals, or even in the same
group. The normal disorganization or death of the tissue re-
sults in clotting ; but there may be death without clotting, as
when the blood is frozen, in various diseases, etc.
To say that fibrin is formed during coagulation expresses in
a crude way a certain fact, or rather the resultant of many
facts. To explain : When gunpowder and certain other ex-
plosives are decomposed, the result is the production of cer-
tain gases. If we knew these gases and their mode of com-
position but in the vaguest way, we should be in much the
same position as we are in regard to the cogulation of the
blood.
There is no difficulty in understanding why the blood does
not clot in the vessels after death so long as they live, nor why
it does coagulate upon foreign bodies introduced into the blood-
stream. So long as it exists under the very conditions under
which it began its being, there is no reason why the blood
should become disorganized (clot). It would be marvelous if
it did clot, for then we could not understand how it could ever
have been developed as a tissue at all. It is just as reasonable
to ask, Why does not a muscle-cell become rigid (clot) in the
body during life ?
Probably in no field in physiology has so much work been
done with so little profit as in the one we are now discussing ;
and. as we venture to think, owing to a misconception of the real
nature of the problem. We can understand the practical im-
portance of determining what circumstances favor coagulation
or retard it, both within the vessels and without them ; but
from a theoretical point of view the subject has been exalted
out of all proportion to its importance.
Coagulation is favored by gentle movement, contact with
foreign bodies, a temperature of about 38° to 40° C, addi-
tion of a small quantity of water, free access of oxygen, etc.
THE BLOOD. 167
The process is retarded by a low temperature, addition of
abundance of neutral salts, extract of the mouth of the leech,
peptone, much water, alkalies, and many other substances.
The excess of carbonic anhydride and diminution of oxygen
seem to be the cause of the slower coagulation of venous blood, ,
hence the blood long remains fluid in animals asphyxiated. A
little reflection suffices to explain the action of most of the fac-
tors enumerated. Any cause which hastens the disintegration
of the blood-cells must accelerate coagulation ; chemical changes
underlie the changes in this as in all other cases of vital action.
Slowing of the blood-stream to any appreciable extent likewise
favors clotting, hence the explanation of the success of the
treatment of aneurisms by pressure. It is plain that in all
such cases the normal relations between the blood and the tis-
sues are disturbed, and, when this reaches a certain point,
death (coagulation) ensues, as with any other tissue.
Clinical and Pathological. — The changes in the blood that
characterize certain abnormal states are highly instructive. If
blood from an animal be injected into the veins of one of an-
other species, the death of the latter often results, owing to non-
adaptation of the blood already in the vessels, and to the tissues
of the creature generally. The corpuscles break up — the change
of conditions has been too great. Deficiency in the quantity of
the blood as a whole (oligcemia) causes serious change in the
functions of the body ; but that a haemorrhage of considerable
extent can be so quickly recovered from speaks much for the
recuperative power of the blood-forming tissues. Various kinds
of disturbances in these blood-forming organs result in either
deficiency or excess of the blood-cells, and in some cases the
appearance of unusual forms of corpuscles.
Ancemia may arise from a deficiency either in the numbers
or the quality of the red cells ; they may be too few, deficient
in size, or lacking in the normal quantity of haemoglobin. In
one form (pernicious ancemia), which often proves fatal in
man a variety of forms in the red blood-cells may appear in the
blood-stream ; some may be very small, some larger than usual,
others nucleated, etc. Again, the white cells may be so multi-
plied that the blood may bear in extreme cases a resemblance
to milk.
In these cases there has been found associated an unusual
condition of the bone-marrow, the lymphatic glands, the spleen,
and, some have thought, of other parts.
168
COMPARATIVE PHYSIOLOGY,
The excessive action of these organs results in the production
and discharge into the blood-current of cells that are immature
and embryonic in character. This seems to us an example of
Fig. 153.
Fig. 149.— Outlines of red corpuscles in a case of profound annemia. 1, 1, normal cor-
puscles; 2, large red corpuscle— megalocyte; 3, 3, very irregular forms— poikilo-
cyt.es; 4, very small, deep-red corpuscles— microcytes.
Fig. 150.— Origin of microcytes from red corpuscles by process of budding and fission.
Specimen from red marrow.
Fig. 151. — Nucleated red blood-corpuscles from blood in case of leukssmia.
Fig. 152.— Corpuscles containing red blood-corpuscles. 1, from blood of child at term;
2, from blood of a leukemic patient.
Fig. 153. — a, 1. 2, 3, spleen-cells containing red blood-corpuscles, b, from marrow; 1,
cell containing nine red corpuscles; 2, cell with reddish granular pigment; 3, fusi-
form cell containing a single red corpuscle, c, connective-tissue corpuscle from
subcutaneous tissue of young rat, showing the intracellular development of red
blood-corpuscles. (Figs. 149-153, after Osier.)
a reversion to an earlier condition. It is instructive also in that
the facts point to a possible seat of origin of the cells in the
adult, and, taken in connection with otber facts, we may say, to
their normal source. These blood-producing organs, having
too much to do in disease, do their work badly — it is incom-
plete.
Althougb the evidence, from experiment, to show that the
THE BLOOD. 169
nervous system in mammals, and especially in man, has an in-
fluence over the formation and fate of the blood generally, is
scanty, there can be little doubt that such is the case, when we
take into account instances that frequently fall under the notice
of physicians. Certain forms of anaemia have followed so di-
rectly upon emotional shocks, excessive mental work and worry,
as to leave no uncertainty of a connection between these and
the changes in the blood ; and the former must, of course, have
acted chiefly if not solely through the nervous system.
It will thus be apparent that the facts of disease are in har-
mony with the views we have been enforcing in regard to the
blood, which we may now briefly recapitulate.
Summary. — Blood may be regarded as a tissue, with a fluid
matrix, in which float cell-contents. Like other tissues, it has
its phases of development, including origin, maturity, and
death. The colorless cells of the blood may be considered as
original imdifferentiated embryo cells, which retain their primi-
tive character ; the non-nucleated red cells of the adult are the
mature form of nucleated cells that in the first instance are
colorless, and arise from a variety of tissues, and which in
certain diseases do not mature, but remain, as they originally
were at first, nucleated. When the red cells are no longer
fitted to discharge their functions, they are in some instances
taken up by amoeboid organisms (cells) of the spleen, liver,
etc.
The chief function of the red corpuscles is to convey oxy-
gen ; of the white, to develop as required into some more differ-
entiated form of tissue, act as porters of food-material, and
probably to take up the work of many other kinds of cells
when the needs of the economy demand it. The fluid matrix
or plasma furnishes the lymph by which the tissues are directly
nourished, and serves as a means of transport for the cells of the
blood.
The chemical composition of the blood is highly complex,
in accordance with the function it discharges as the reservoir
whence the varied needs of the tissues are supplied ; and the
immediate receptacle (together with the lymph) of the entire
waste of the body ; but the greater number of substances exist
in very minute quantities. The blood must be maintained of
a certain composition, varying only within narrow limits, in
order that neither the other tissues nor itself may suffer.
The normal disorganization of the blood results in coagula-
170 COMPARATIVE PHYSIOLOGY.
tion, by which, a substance, proteid in nature, known as fibrin,
is formed, the antecedents of which are probably very variable
throughout the animal kingdom, and are likely so even in the
same group of animals, under different circumstances ; and a
substance abounding in proteids (as does also plasma), known
as serum, squeezed from the clot by the contracting fibrin. It
represents the altered plasma.
Certain well-known inorganic salts enter into the composi-
tion of the blood — both plasma and corpuscles — but the princi-
pal constituent of the red corpuscles is a pigmented, ferrugi-
nous proteid capable of crystallization, termed haemoglobin. It
is respiratory in function.
THE CONTEACTILE TISSUES.
That contractility, which is a fundamental property in some
degree of all protoplasm, becoming' pronounced and definite,
giving rise to movements the character of which can be pre-
dicted with certainty once the form of the tissue is known, finds
its highest manifestation in muscular tissue.
Very briefly, this tissue is made up of cells which may be
either elongated, fusiform, nucleated, finally striated lengthwise,
Fig. 154.
Fig. 155.
Fig. 154.— Muscular fibers from the urinary bladder of the human subject. 1 x 200
(Sappey.) 1,1,1, nuclei; 2,2,2. borders of some of the fibers; 3,3, isolated fibers;
4, 4, two fibers joined together at 5.
Fig. 155.— Muscular fibers from the aorta of the calf. 1 x 200. (Sappey.) 1, 1, fibers
joined with each other; 2, 2, 2, isolated fibers.
but non-striped transversely, united by a homogeneous cement
substance, the whole constituting non-striped or involuntary
172 COMPARATIVE PHYSIOLOGY.
muscle ; or, long nucleated fibers transversely striped, covered
with an elastic sheath of extreme thinness, bound together
into small bundles by a delicate connective tissue, these again
into larger ones, till what is commonly known as a " muscle "
is formed. This, in the higher vertebrates, ends in tough, ine-
lastic extremities suitable for attachment to the level's it may be
required to move (bones). Certain of the tissues will be found
briefly described in the sections preceding " Locomotion."
Comparative. — The lowest animal forms possess the power
of movement, which, as we have seen in Amoeba, is a result
rather of a groping after food ; and takes place in a direction
it is impossible to predict, though no doubt regulated by laws
definite enough, if our knowledge were equal to the task of de-
fining them.
Those ciliary movements among the infusorians, connected
with locomotion and the capture of food, are examples of a
protoplasmic rhythm of wonderful beauty and simplicity.
Muscular tissue proper first appears in the Coelenterata, but
not as a wholly independent tissue in all cases. In many
ccelente rates cells exist, the lower part of which alone forms a
delicate muscular fibei', while the superficial portion (myoblast),
composing the body of the cell, may be ciliated and is not con-
tractile in any special sense. The non-striped muscle-cells are
most abundant among the in-
..-—"•-••,.,-. . ..,-•; :"■'■>'' vertebrates, though found in
,.^/ the viscera and a few other
parts of vertebrates. This
: - -J"^ /;'jg_2,_ form is plainly the simpler
and more primitive. The
/ voluntary muscles are of the
ri^^r^£SfaSaJelly-fiSh'the^ Griped variety in articulates
and some other invertebrate
gi*oups and in all vertebrates ; and there seems to be some re-
lation between the size of the muscle-fiber and the functional
power of the tissue — the finer they are and the better supplied
with blood, two constant relations, the greater the contrac-
tility.
Whether a single smooth muscle-cell, a striped fiber (cell),
or a collection of the latter (muscle) be observed the invariable
result of contraction is a change of shape which is perfectly
definite, the long diameter of the cell or muscle becoming
shorter, and the short diameter longer.
THE CONTRACTILE TISSUES.
173
Ciliary Movements. — This subject has been already con-
sidered briefly in connection with some of the lower forms of
life presented for study.
It is to be noted that there is a gradual replacement of this
form of action by that of muscle as we ascend the animal
scale ; it is, however, retained even in the highest animals in
the discharge of functions analogous to those it fulfills in the
invertebrates.
Thus, in Vorticella, we saw that the ciliary movements of
the peristome caused currents that carried in all sorts of parti-
cles, including food. In a creature so high in the scale as the
frog we find the alimentary tract ciliated ; and in man himself
a portion of the respiratory tract is provided with ciliated cells
concerned with assisting gaseous interchange, a matter of the
highest importance to the well-being of the mammal. As be-
fore indicated, ciliated cells are found in the female generative
organs, where they play a part already explained.
It is a matter of no little significance from an evolutionary
point of view, that cil-
iated cells are more
widely distributed in
the foetus than in the
fully developed ani-
mal.
As would be ex-
pected the movements
of cilia are affected by
a variety of circum-
stances and reagents ;
thus, they are quick-
ened by bile, acids,
alkalies, alcohol, ele-
vation of temperature
up to about 40° C,
etc. ; retarded by cold, '
carbonic anhydride, Fk
ether, chloroform, etc.
In some cases their
action may be arrested
and re-established by
treatment with rea-
gents, or it may recommence without such assistance.
. 157.— Nodes of Ranvier and lines of Fromann
(Kanvier). A. Intercostal nerve of the mouse,
treated with silver nitrate. B. Nerve-fiber from
the sciatic nerve of a full-grown rabbit. A, node
of Ranvier ; J/, medullary substance rendered
transparent by the action of glycerin: CY, axis-
cylinder presenting the lines of Fromann, which
are very distinct near the node. The lines are less
marked at a distance from the node.
All this
174
COMPARATIVE PHYSIOLOGY.
seems to point to ciliary action as falling under the laws gov-
erning the movements of protoplasm in general. It is impor-
tant to bear in mind that ciliary action may go on in the cells
of a tissue completely isolated from the animal to which it he-
longs, and though influenced, as just explained, by the sur-
roundings, that the movement is essentially automatic, that is,
independent of any special stimulus, in which respect it differs
a good deal from voluntary muscle, which usually, if not al-
ways, contracts only when stimulated.
The lines along which the evolution of the contractile tissues
has proceeded from the indefinite out do wings and withdrawals
of the substance of
Amoeba up to the
highly specialized
movements of a
striped muscle-cell
are not all clearly
marked out ; but
even the few facts
mentioned above
suffice to show gra-
dation, intermedi-
ate forms. A sim-
ilar law is involved
in the muscular
contractility mani-
fested by cells with
other functions.
The automatic (self -
Fig. 158.— Mode of termination of the motor-nerves (Flint,
after Rouget). A. Primitive fasciculus of the thyro-
hyoid muscle of the human subject, and its nerve-
tube : 1,1, primitive muscular fasciculus; 2, nerve-
tube ; 3, medullary substance of the tube, which is
seen extending to the terminal plate, where it disap-
pears ; 4, terminal plate situated beneath the sarco-
femma— that is to say, between it and the elementary
fibrilhe; 5, 5, sarcolemma. B. Primitive fasciculus of
the intercostal muscle of the lizard, in which a nerve- Originated, mcle-
tube terminates: 1,1, sheath of the nerve-tube: 2, T,0„.i„„+ l.j,,™^!^ ^t
nucleus of the sheath; 3, 3, sarcolemma becoming peuueiii/ iciigei^ ui
continuous with the sheath; 4, medullary substance „ cti*miln<A rhvthm
of the nerve-tube, ceasing abruptly at the site of the a SUmiUUS; lnymm
terminal plate; 5, 5, terminal plate; 6. 6, nuclei of the suefffestive of cilia-
plate; 7, 7. granular substance which forms the princi- oa
pal element of the terminal plate and which is con- ry movement, more
tinuous with the axis cylinder; 8, 8, undulations of _„Q •!•--. i • it,,
the sarcolemma reproducing those of the fibrilhe; mamiehi ui ui t;
0, 9, nuclei of the sarcolemma. earlier developed
smooth muscle than in the voluntary striped muscle of higher
vertebrates, indicating further by the regularity with which
certain organs act in which this smooth muscular tissue is pre-
dominant, a relationship to ciliary movement something in
common as to origin — in a word, an evolution. And if this be
borne in mind, we believe many facts will appear in a new
THE CONTRACTILE TISSUES.
175
light, and be invested with a breadth of meaning they would
not otherwise possess.
The Irritability of Muscle and Nerve. — An animal, as a frog,
deprived of its brain, will remain motionless till its tissues have
died, unless the animal be in some way stimulated. If a mus-
cle be isolated from the body with the nerve to which it be-
longs, it will also remain passive ; but, if an electric current be
passed into it, if it be pricked, pinched, touched with a hot body
or with certain chemical reagents, contraction ensues ; the same
happening if the nerve be thus treated instead of the muscle.
The changes in the muscle and the nerve will be seen later to
have much in common ; the muscle alone, however, contracts,
undergoes a visible change of form.
Fig. 159.— Intrafibrillar terminations of the motor nerve in striated muscle, stained
with gold chloride (Landois).
Now, the agent causing this is a stimulus, and as we have
seen, may be mechanical, chemical, thermal, electrical, or nerv-
ous. As both nerve and muscle are capable of being function-
ally affected by a stimulus, they are said to be irritable ; and
since muscle does not contract without a stimulus, it is said to
be non-automatic.
Now, since muscle is supplied with nerves, as well as blood-
vessels, which end in a peculiar way {end plates) beneath the
muscle-covering (sarcolemma) in the very substance of the pro-
toplasm, it might be that when muscle seemed to be stimu-
lated, as above indicated, the responsive contraction was really
due to the excited nerve terminals ; and thus has arisen the
question, Is muscle of itself really irritable ?
What has been said as to the origin of muscular tissue points
very strongly to an affirmative answer, though it does not fol-
low that a property once possessed in the lower forms of a tissue
may not be lost in the higher. From various facts it may be
concluded that muscle possesses independent irritability.
THE GKAPHTC METHOD AND THE STUDY OF
MUSCLE PHYSIOLOGY.
It is impossible to study the physioloay of muscle to the best
advantage without the employment of the graphic method ;
and, on the other hand, no
tissue is so well adapted for
investigation by the isolated
method — i. e., apart from the
animal to which it actually
belongs — as muscle ; hence
the convenience of introduc-
ing at an early period our
study of the physiology of
contractile tissue and illus-
trations of the graphic meth-
od, the general principles of
which have already been
considered.
The descriptions in the
text will be brief, and the
student is recommended to
examine the figures and ac-
companying explanations
with some care.
Chronographs, Revolving
Cylinders, etc.— Fig. 160 rep-
resents one of the earliest
Fin. 100.- -Original chronometer, devised by „ „ ,-,
Thomas young, for measuring minute forms ot apparatus tor tUe
portions of time (after McKendrick). a, , e i ■ <• •,+.„
cylinder revolving on vertical axis; b measurement of brief mter-
weight acting as motive power; e,d, small ,,„ic nf ti-me> pm-isnstiTiP' of a
balls for regulating the velocity of the vals or time> consisting 01 d
cylinder; e, marker recording a line on simple mechanism for pro-
ducing the movement of a
cylinder, which may be covered with smoked paper, or other-
THE STUDY OF MUSCLE PHYSIOLOGY.
177
wise prepared to receive impressions made upon it by a point
and capable of being raised or lowered, and its movements reg-
C. ,1
I.
1
J.
Fig. 161. — Myographic tracing, such as is obtained when the cylinder on which it is
written does not revolve during the contraction of the muscle (after McKendrlck).
ulated. The cylinder is ruled vertically into a certain number
of spaces, so that, if its rate of revolution is known and is con-
stant (very important), the length of time of any event recorded
on the sensitive surface may be accurately known. This whole
apparatus may be considered a chronograph in a rough form.
But a tuning-fork is the most reliable form of chronograph,
provided it can be kept in coustant action so long as required ;
Fig. 162. — Marty's chronograph as applied to revolving cylinder (after McKendrick).
a, galvanic element; b, wooden stand bearing tuning-fork (two hundred vibrations
per second); c, electro-magnet between limbs of tuning-fork: il. e. positions for
tuning-forks of one hundred and fifty vibrations per second; /. tuning-fork lying
loose, which may be applied to d\ g, revolving cylinder: h. electric chronograph
kept in vibration synchronous with the tuning-fork interrupter. The current
working the electro-magnet from a, is interrupted at i. Foucaulfs regulator is
seen over the clock-work of the cylinder, a little to the right of cj.
12
178
COMPARATIVE PHYSIOLOGY.
and is provided with a recording apparatus that does not cause
enough friction to interfere with its vibrations.
Fig. 162 illustrates one arrangement that answers these con-
ditions fairly well.
The marker, or chronograph, in the more limited sense, is
kept in automatic action by the fork interrupting the current
from a battery at a certain definite rate answering to its own
proper note.
Marey's chronograph, which is represented at h above, and
in more detail below, in Fig. 163, consists of two electro-mag-
nets armed with keepers, between which is the writer, which
Fio. 163.— Side view of Marey's chronograph (after McKendriek). a, a, coils of wire;
b, b, keepers of electro-magnets; c, vibrating style fixed to the steel plate e; d,
binding screws for attachment of wires; + from interrupting tuning-fork; — to
the battery.
has a little mass of steel attached to it, the whole working in
unison with the tuning-fork, so that an interruption of the cur-
rent implies a like change of position of the writing-style, which
is always kept in contact with the recording surface.
Fig. 173 shows the arrangements for recording a single
muscle contraction, and
Fig. 174 the character of
the tracing obtained.
A muscle-nerve prepa-
ration, which usually con-
sists of the gastrocnemius
of the frog with the sciatic
nerve attached, clamped by
Fig. 164.— Muscle-nerve preparation, showing a portion of the femur cut
gastrocnemius muscle, sciatic nerve, and „. ... ,, .
portion of femur of frog, for attachment Oil With the muscle, IS
to a vise (after Rosenthal). made> Qn stimulatioilj to
THE STUDY OF MUSCLE PHYSIOLOGY.
179
raise a weighted lever which is attached to a point writing on a
cylinder moved by some sort of clock-work. In this case the
cylinder is kept stationary during the contraction of the mus-
cle ; hence the records appear as straight vertical lines.
For recording movements of great rapidity, so that the in-
tervals between them may be apparent, such an apparatus as is
Fig. 165. — Spring myograph of Du Bois-Rcymond (after Rosenthal). The arrange-
ments for registering various details are similar to those for pendulum myograph
(Fig. 173).
figured here (Fig. 165) answers well, the vibrations of a tuning-
fork being written on a blackened glass plate, shot before a chro-
nograph by releasing a spring.
Several records may be made successively by more compli-
cated arrangements, as will be explained by another figure later.
THE APPARATUS USED FOR THE STIMULATION OF
MUSCLE.
It is not only important that there should be accurate and
delicate methods of recording muscular contractions, but that
there be equally exact methods of applying, regulating, and
measuring the stimulus that induces the contraction.
Fig. 166 gives a representation of the inductorium of Du
Bois-Reymond, by which either a single brief stimulation or
a series of such repeated with great regularity and frequency
180
COMPARATIVE PHYSIOLOGY.
Fig. 166. — Du Bois-Reymond's inductorium (after Rosenthal), i, secondary coil; c,
primary coil; b, electro-magnet; A, armature of hammer; /, small movable screw.
The current from battery, ascending metal pillar, passes along hammer, and by
6crew gets into primary coil, thus inducing current in secondary coil. By con-
nection between primary coil and wires around soft iron of b, iron becomes a mag-
net, hammer is attracted from screw/, and current thus broken; but when this
occurs, soft iron ceases to be a magnet necessarily, and, hammer springing back,
the whole course of events is repeated. This may occur several hundred times in
a second. The above may be clearer from diagram, Fig. 167. By sliding second-
ary coil up and down, strength of induced current can be graduated.
may be effected. The apparatus consists essentially of a pri-
mary coil, secondary coil, magnetic interrupter, and a scale
Fig. 167.— Diagrammatic representation of the working of Fig. 166 (after Rosenthal)
THE STUDY OF MUSCLE PHYSIOLOGY.
181
to determine the relative strength of the current employed.
The instrument is put into action by one or more of the various
well-known galvanic cells, of which Daniell's are suitable for
most experiments.
Fig. 169.
Fig. 168.
Fig. 168.— Pflfiger's myograph. The muscle may be fixed to the vise C in the moist
chamber, the vise connecting with the lever E E, the point of which touches the
plate of smoked glass G. The lever is held in equipoise by //. When weights are
placed in scale-pan F, the lever writes the degree of extension effected (after Ro-
senthal).
Fig. 169. — Tetanizing key of Du Bois-Iteymond (after Rosenthal). Wires may be
attached at b and c. When d is down the current is "short-circuited," i. e.. does
not pass through the wires, but direct from c through d to b. or the reverse, since
6, c, d are of metal, and, on account of their greater cross-section, conduct so
much more readily than the wires, a is an insulating plate of ebonite. This form
of key is adapted for attachment to a table, etc.
The access to, or exclusion of the current from, the indue-
torium is effected by some of the forms of keys, a specimen of
which is illustrated in Fig. 169.
The moist chamber, or some other means of preventing the
drying of the preparation, which would soon result in impaired
182
COMPARATIVE PHYSIOLOGY.
action, followed by death, is essential. A moist chamber con-
sists essentially of an inclosed cavity, in which is placed some
wet blotting-paper, etc., and is usually made with glass sides.
The air in such a chamber must remain saturated with moisture.
A good knowledge of the subject of electricity is especially
valuable to the student of physiology. But there are a few ele-
mentary facts it is absolutely necessary to bear in mind : 1. An
induced current exists only at the moment of making or break-
ing a primary (battery) current. 2. At the moment of making,
the induced current is in the opposite direction to that of the
primary current, and the reverse at breaking. 3. The strength
of the induced current varies with the strength of the primary
current. 4. The more removed the secondary coil from the
primary the weaker the current (induced) becomes.
The clock-work mechanism and its associated parts, as seen
in Fig. 170, on the right, is usually termed a myograph.
Fig. 170.— Arrangement of apparatus for transmission of muscular movement by tam-
bours (after McKendrick). a, galvanic element; b, primary coil; c, secondary coil
of inductorium; d, metronome for interrupting primary circuit when induction
current is sent to electrodes k\ h, forceps for femur; the muscle, which is not
here represented, is attached to the receiving tambour g, by which movement is
transmitted to recording tambour e, which writes on cylinder/.
Instead of muscular or other movements being communi-
cated directly to levers, the contact may be through columns
of air, which, it will be apparent, must be capable of communi-
cating very slight changes if the apparatus responds readily to
the alterations in volume of the inclosed air.
Fig. 171 represents a Marey's tambour, which consists essen-
THE STUDY OF MUSCLE PHYSIOLOGY.
183
Fig. 171. — Tambour of Marey (after McKendrick). a, metallic case; b, thin India-rub-
ber membrane; c, thin disk of aluminium supporting lever d, a small portion of
which only is represented; e, screw for placing support of lever vertically over'c;
/j metallic tube communicating with cavity of tambour for attachment to an In-
dia-rubber tube.
tially of a rigid metallic case provided with an elastic top, to
which a lever is attached, the whole being brought into com-
munication with a column of air in an elastic tube. The work-
ing of such a mechanism will be evident from Figs 170 and 172.
1
Fig. 172.— Tambours of Marey arranged for transmission of movement (after McKen-
drick). a, receiving tambour; b, India-rubber tube; c, registering tambour; d,
spiral of wire, owing to elasticity of which, when tension is removed from a, the
lever ascends.
The greatest danger in the use of such apparatus is not fric-
tion but oscillation, so that it is possible that the original move-
ment may not be expressed alone or simply exaggerated, but
also complicated by additions, for which the apparatus itself is
responsible.
184
COMPARATIVE PHYSIOLOGY.
Fih. 173.
THE STUDY OF MUSCLE PHYSIOLOGY. 185
Fig. 173.— Diagrammatic representation of the pendulum myograph. The smoked-
glass plate. A, swings with a pendulum, B. Before an experiment is commenced
the pendulum is raised up to the right and kept in position by the tooth, a. catch-
ing on the spring-catch, b. On depressing the catch, b, the glass plate being set
free swings into the new position indicated by the dotted lines, and is held there
by the tooth, «', meeting the catch, b' . In the course of its swing the tooth, a,
coming into contact with the projecting steel rod, c, knocks it to one side, into
the position indicated by the dotted line, c'. The rod, c, is in electric continuity
with the wire, x, of the"primary coil of an induction machine. In like manner
the screw, d, is in electric continuity with the wire, y, of the same primary coil.
The screw, d, and the rod, c. are provided with platinum points, and both are in-
sulated by means of the ebonite block, e. The circuit of the primary coil to which
.rand y belong is closed as long as c and d are in contact. When in its swing
the tooth. «', knocks c away from d, the circuit is immediately broken, and a
•' breaking" shock is sent through the electrodes connected with the secondary
coil of the machine, and so through the nerve. A lever is brought to bear on the
glass plate, and when at rest describes an arc of a circle of large radius. The tun-
ing-fork,/(ends only seen), serves to mark the time (after Foster).
Apparatus of this kind is not usually employed much for
experiments with muscle ; such an arrangement is, however,
showm in Fig. 170, in which also will be seen a metronome, the
pendulum of wdiich, by dipping into cups containing mercury,
makes the circuit. Such or a simple clock may be utilized for
indicating the longer intervals of time, as seconds.
A SINGLE SIMPLE MUSCULAR CONTRACTION.
Experimental Facts.— The phases in a single twitch or mus-
cular contraction may be studied by means of the pendulum
myograph (Fig. 173). It consists of a heavy pendulum, which
swings from a position on the right to a corresponding one on
the left, where it is secured by a catch. During the swing of
the pendulum, which carries a smoked-glass plate (by means
of arrangements more minutely described below the figure), a
tuning-fork writes its vibrations on the plate, on which is in-
scribed the marking indicating the exact moment of the break-
ing of an electric current, which gives rise to a muscle contrac-
tion that is also recorded on the plate.
The tracing on analysis presents : 1. The record of a tuning-
fork making one hundred and eighty vibrations in a second.
2. The parallel marking of the lever attached to the muscle
before it began to rise. 3. A curve, at first rising slowly, and
then rapidly to a maximum. 4. A cmwe of descent similar in
character, but somewhat more lengthened.
We may interpret this record somewhat thus : 1. A rise of
the lever answering to the shortening of the muscle to which it
is attached following upon the momentary induction shock,
as the entrance of the current into the nerve, the stimulation of
which causes the contraction, may be called. 2. A period before
186 COMPx\RATIVE PHYSIOLOGY.
the contraction begins, which, as shown by the time marking,
occupies in this case — - , or about TV of a second. In the tracing
the upward curve indicates that the contraction is at first rela-
tively slow, then more rapid, and again slower, till a brief sta-
a b
Fig. 174.— Muscle-curve obtained by the pendulum myograph (Foster). Read from
left to right. The latent period is indicated by the space between a and b, the
length of which is measured by the waves of a tuning-fork, making one hundred
and eighty double vibrations in a second; and in like manner the duration of the
other phases of the contraction may be estimated.
tionary period is reached, when the muscle gradually but rap-
idly returns to its previous condition, passing through the same
phases as during contraction proper. In other words, there is
a period of rising and of falling energy, or of contraction and
relaxation. 4. A period during which invisible changes, as
will be explained later, are going on, answering to those in the
nerve that cause the molecular commotion in muscle which
precedes the visible contraction — the latent period, or the period
of latent stimulation.
The facts may be briefly stated as follows : The stimulation
of a muscle either directly or through its nerve causes contrac-
tion, followed by relaxation, both of which are preceded by a
latent period, during which no visible but highly important
molecular changes are taking place. The whole change of events
is of the briefest duration, and is termed a muscle contraction.
The tracing shows that the latent period occupied rather more
than Tfs second, the period of contraction proper about T^, and
of relaxation ^ second, so that the whole is usually begun and
ended within TV second ; yet, as will be learned later, many
chemical and electrical phenomena, the concomitants of vital
change, are to be observed.
In the case just considered it was assumed that the muscle
THE STUDY OF MUSCLE PHYSIOLOGY.
1S7
was stimulated through its nerve. Precisely the same results
would have followed had the muscle been caused to contract by
the momentary application of a chemical, thermal, or mechanical
stimulus.
If the length of nerve between the point of stimulation and
the muscle was considerable, some difference would be observed
Fig. 175. — Diagrammatic representation of the measurement of velocity of nervous
impulse (Foster). Tracing taken by pendulum myograph (Fig. 173). The nerve
of same muscle-nerve preparation is stimulated in one case as far as possible from
muscle, in the other as near to it as possible. Latent period is ab, ab'. respect-
ively. Difference between ab and ab' indicates, of course, length of time occu-
pied by nervous impulse in traveling along nerve from distant to near point.
in the latent period if in a second case the nerve were stimu-
lated, say, close to the muscle. This is represented in Fig. 175,
in which it is seen that the latent period in* the latter case is
shortened by the distance from b' to &, which must be owing
to the time required for those molecular changes which, occur-
ring in a nerve, give rise to a contraction in the muscle to which
it belongs ; in fact, we have in this method the means of estimat-
ing the rate at which these changes pass along the nerve — in
other words we have a means of measimng the speed of the
propagation of a nervous impulse. The estimated rate is for the
frog twenty-eight metres per second, and for man about thirty-
three metres. As the latter has been estimated for the nerve,
with its muscle in position in the living body, it must be re-
garded rather as a close approximation than as exact as the
other measurements referred to in this chapter.
It will be borne in mind that the numbers given as repre-
senting the relative duration of the events vary with the ani-
mal, the kind of muscle, and a variety of conditions affecting
the same animal.
TETANIC CONTRACTION.
It is well known that a weight may be held by the out-
stretched arm with apparently perfect steadiness for a few
188
COMPARATIVE PHYSIOLOGY.
seconds, but that presently the arm begins to tremble or vi-
brate, and soon the weight must be dropped. The arm was
maintained in its position by the joint contraction of several mus-
cles, the action of which might be described (traced) by a writer
attached to the hand and recording on a moving surface. Such
a record would indicate roughly what had happened ; but the
exact nature of a muscular contraction in such a case can best be
learned by laying bare a single muscle, say in the thigh of a
frog, and arranging the experiment so that a graphic record
shall be made.
Using the apparatus previously described (Fig. 173), a series
of induction shocks may be sent into the muscle with the result
indicated in Figs. 176 and 177, according to the rate of interrup-
tion of the current.
j*
1**
^^w»*''*iw~*1* ' — \
Fig. 176.— Curve of imperfect, tetanic contraction (Foster). Uppermost tracing indi-
cates contractions of muscle; intermediate, when the shocks were given; lower,
time-markings of intervals of one second. Curve to be read, like others, from left
to right, and illustrates at the end a ."'contraction remainder."
If the stimuli follow each other with a certain rapidity, such
a tracing as that represented in Fig. 176 is obtained ; and if the
rapidity of the stimulation exceeds a fixed rate, the result is that
seen in Fig. 177.
Fro. li'T.— Curve of complete tetanic contraction (Foster).
THE STUDY OP MUSCLE PHYSIOLOGY. 189
It is possible to see in these tracings a genetic relation, the
second figure being evidently derivable from the first, and the
third from the second, by the fusion of all the curves into one
straight line.
The Muscle Tone. — There are a number of experimental facts
from which the conclusion has been drawn that tetanic contrac-
tion is accompanied by a muscle tone whioii is in itself evidence
of the nature of the contraction.
We may safely conclude that, at all events, most of the mus-
cular contractions occurring within the living body are tetanic
— i. e., the muscle is in a condition of shortening, with only very
brief and slight phases of relaxation ; and that a comparatively
small number of individual contractions suffice for tetanus
when caused by the action of the central nervous system;
though, as proved by experiments on muscle removed from the
body, they may be enormously increased. While a few stimu-
lations per second suffice to cause tetanus, it will also persist
though thousands be employed.
THE CHANGES IN A MUSCLE DURING CONTRACTION.
Though the change in form is very great during the con-
traction of a muscle, the change in bulk is almost inappreci-
able, amounting to a diminution of not more than about T-gVo
of the volume. In fact, according to the latest investigator,
there is no diminution whatever.
Since the fibers of striped muscle are of very limited length
(30 to 40 mm.), it would seem that a contraction originating in
one fiber must be capable of initiating a similar action in its
neighbor ; and, as the ends of the fibers lie in contact, it is easy
to understand how the wave of contraction spreads. Normally,
the contraction must pass from about the center of the muscle-
cell where the nerve terminates in the end-plate.
THE ELASTICITY OF MUSCLE.
In proportion as bodies tend to resume their original form
when altered by mechanical force are they elastic, and the ex-
tent to which they do this marks the limit of their elasticity.
If a muscle (best one with bundles of fibers of about equal
length and parallel arrangement) be stretched by a weight
attached to one end, it will, on removal of the extending force,
190
COMPARATIVE PHYSIOLOGY.
return to its original length ; and if a series of weights which
differ by a common increment be applied in succession and the
degrees of extensions compared, as may be
done by the graphic method, it will be ap-
parent that the increase in the extension
does not exactly correspond with incre-
mnent in the weight, but is proportionally
less. With an inorganic body, as a watch-
spring, this is not the case.
Further, the recoil of the muscle after
the removal of the weight is not perfect
for all weights ; but within certain narrow
limits this is the case, i. e., the elasticity
of muscle, though slight (for it is easily
over-extended), is perfect. When once a
muscle is over-extended, so weighted that
it can not reach its original length almost
at once, it is very slow to recover, which
explains the well-known duration of the
effects of sprains, no doubt owing to some
profound molecular change associated with
the stretching.
The tracings below show at a glance
the difference between the elasticity of
muscle and of ordinary bodies.
It is a curious fact that a muscle during
the act of contraction is more extensible
than when passive ; a disadvantage from
a purely physical point of view, but prob-
ably a real advantage as tending to obviate
tey. sprain by preventing too sudden an appli-
mond's apparatus for cation of the extending force,
tension in muscle (after It will be borne in mind that the limbs
SrSd)" at?ahchldadtUo are held together as by elastic bands slight
mitnCaeien8° be observed ly on the stretch, owing to the elasticity
of the muscles. Now, as seen in many
tracings of muscular contraction, there is a tendency to imper-
fect relaxation after contraction — the contraction remainder
or elastic after-effect, which can be overcome by gentle trac-
tion. In the living body, the weight of the limbs and the action
of the stretched muscles on the side of the limb opposite to that
on which the muscles in actual contraction are situated, com-
THE STUDY OF MUSCLE PHYSIOLOGY. 191
bine to make the action of the muscle more perfect by over-
coming this tendency to imperfect relaxation, which is proba-
Fig. 179.— Illustrations of the difference in elasticity of inanimate and living matter
(after Yeo). 1. Shows graphically behavior of a steel spring under equal incre-
ments of weight. 2. A similar tracing obtained from an India-rubber band. 3.
The same from a frog's muscle. Note that the extension decreases with equal in-
crements of weight, and that the muscle fails to return to the original position
(abscissa) after removal of the weight.
bly less marked, independent of these considerations, in the
living body. This elasticity of living muscles, which is com-
pletely lost on death, is a fair measure of their state of health
or organic perfection. Hence that hard (elastic recoil) feeling
of the muscles in young and vigorous persons, especially ath-
letes, in whom muscle is brought to the highest degree of per-
fection.
This property is then essentially the outcome of vitality,
which is in a word the foundation of the differences noted be-
tween the elasticity of inorganic and organic bodies. A mus-
cle, the nutrition of which is suffering from whatever cause,
whether deficient blood-supply, fatigue, or actual disease, is
deficient in elasticity. We wish to emphasize these relations,
for we consider it very important to avoid regarding vital phe-
nomena in the light of physics merely, which the employment
of the graphic method (and indeed all methods by which we re-
move living things out of their normal relations) fosters.
Electrical Phenomena of Mnscle. — The contraction and
probably the resting stage of muscle are attended by the gen-
eration of electrical currents, the direction of which is indicated
in Fig. 180.
It will be observed that the diagram indicates that between
no current and the strongest obtainable there are all shades of
192
COMPARATIVE PHYSIOLOGY.
Fig. 180.— Representation of electrical currents in a muscle-rhombus (after Rosenthal).
strength, according to the parts of the muscle connected by the
electrodes. The strongest is that resulting when the superfi-
cial equator and the transverse center are connected ; and it is
found that the nearer these points are approached the stronger
the current becomes.
It is important to note that the electric current of muscle,
however viewed, is associated with the chemical and all the
other molecular changes of which the actual contraction is
but the outward and visible sign ; and since the currents have
an appreciable duration, wane with the vitality of the tissue,
and wholly disappear at death, they must be associated with the
fundamental facts of organic life ; for it is to be remembered
that electrical currents are not confined to muscle, but have
been detected in the developing embryo, and even in vegetable
protoplasm. Though the evidence is not yet complete, it seems
likely that electrical phenomena may prove to be associated
with (we designedly avoid any more definite expression) all
vital phenomena.
Chemical Changes in Muscle.— At a variable period after
death the muscles become rigid, producing that stiffness {rigor
mortis) so characteristic of a recent cadaver.
THE STUDY OP MUSCLE PHYSIOLOGY. 193
The subject can be studied in some of its aspects to great ad-
vantage in an isolated individual muscle.
Three changes in a muscle that has passed into death rigor
are constant and pronounced. The living muscle, either alka-
line or neutral in reaction, has become decidedly acid ; an
abundance of carbonic anhydride is suddenly given off ; and
myosin, a specific proteid, has been formed. That these phe-
nomena have some indissoluble connection with each other so
far as the first two at least are concerned, while not absolutely
certain, seems probable, as will be learned shortly.
It will be borne in mind that muscle-fibers are tubes con-
taining semifluid protoplasm, and that a coagulation of the lat-
ter must give rise to general rigor. This protoplasmic substance
can be extracted at a low temperature from the muscles of the
frog, and, as the temperature rises, coagulates like blood, giving
rise to a clot (myosin) and muscle-serum, a fluid not very unlike
the serum of blood.
This myosin can also be extracted from dead rigid muscles
by ammonium chloride, etc. It resembles the globulins gen-
erally, but is less soluble in saline solutions than the globulin
of blood (paraglobulin) ; is less tough than fibrin ; has a very
low coagulating point (55° to 60° C.) ; and is somewhat jelly-
like in appearance. The clotting of blood and of muscle is thus
analogous, myosin answering to fibrin, and there being a serum
in each case, both processes marking the permanent disorgani-
zation of the tissue. The reaction seems to be due to the forma-
tion of a kind of lactic acid, probably sarcolactic ; though
whether due to excessive production of this acid, on the death
of the muscle, which for some reason does not remain free in
the living muscle, or whether sarcolactic acid arises as a new
product, is uncertain. It is certain that the acid reaction of
dead muscle is not owing to carbonic acid, for the reddened
litmus does not change color on drying.
That a muscle in action does use up oxygen and give off
carbonic anhydride can be definitely proved ; though it is
equally clear that the life of a muscle is not dependent on a
constant supply of oxygen as is that of the individual, for a
muscle can live, even contract long and vigorously, in an atmos-
phere free from this gas, as in nitrogen.
From the suddenness of the increase of carbonic anhydride,
the onset of death and rigor mortis has been compared to an
explosion.
13
194 COMPARATIVE PHYSIOLOGY.
After this the muscle becomes greatly changed physically ;
its elasticity and translucency are lost ; there is absence of
muscle-currents ; it is wholly unheritable, is less extensible — it
is, as before stated, firmer — it is dead.
But these fundamental phenomena, the increase of carbonic
anhydride and the acid reaction, are observable after prolonged
tetanus. It was, therefore — putting all the facts together that
we now refer to and others, not forgetting that a muscle is
always respiring, inhaling oxygen, and exhaling carbonic an-
hydride — not um^easonable to conclude that normal tetanus
and rigor mortis were but exaggerated conditions of a natural
state. The coagulation of the muscle protoplasm {plasma),
giving rise to myosin, was, however, a serious obstacle to the
adoption of this view. But it has very recently been urged
with great plausibility that an old view is correct, viz., that
rigor mortis (contracture) is the last act of muscle-life ; it is, in
fact, a prolonged tetanus or contracture, ending in most cases,
though not all, in coagulation of the myosin. This state can
be induced and recovered from in favorable cases by cutting
off the blood from a part by ligature, and later readmitting it
to the starving region. It has been suggested that the prod-
ucts of the muscle-waste, usually washed away by the blood-
stream, in such an experiment and after death, collect and act
as a stimulant to the muscle, causing it to remain in permanent
contraction.
The other constituents of dead muscle and their relative
properties may be learned from the following table (Von Bibra):
Water 744'5
Solids : Myosin, elastic substance, etc., in-
soluble in water 155*4
Soluble proteids 19*3
Gelatin 207
Extractives and salts 37'1
Fats 230
255-5— 255-5
Total 1,000
Among the extractives of muscle very important is creatin
(-2 to '3 per cent), a nitrogenous crystalline body. Certain
allied forms, as xanthin, hypoxanthin (sarkin), carnin, taurin,
and uric acid, are also found.
Glycogen (animal starch), very abundant in all the tissues,
THE STUDY OF MUSCLE PHYSIOLOGY. 195
including the muscles of the embryo, is found in small quantity
in the muscles of the adult; and in the heart-muscle a peculiar
sugar (inosit) is present.
It is, of course, very difficult to say to what extent the bodies
known as extractives exist in living muscle, though that glyco-
gen, fats, and certain salts are normally present admits of little
doubt.
There is a coloring matter in muscle, more abundant in the
red muscles of certain animals than the pale, allied to haemo-
globin, if not identical with that body.
It may be stated as a fact, the exact significance of which
is unknown, that during contraction the extractives soluble in
water decrease, while those soluble in alcohol increase.
It will, however, be very plain, from what has been stated
in this section, that life processes and chemical changes are
closely associated, and to realize this is worth much to the
student of Nature.
THERMAL CHANGES IN THE CONTRACTING MUSCLE.
Since very marked chemical changes accompany muscular
contraction, it might be expected that there would be some
modification in temperature, and probably in the direction of
elevation. Experiment proves this to be the case.
But why should a muscle when at rest, as may be shown,
maintain a certain temperature, unless chemical changes are
constantly taking place ? As already stated, such is the case,
and the rise on passing into tetanus is simply an expression of
increased chemical action.
No machine known to us resembles muscle except super-
ficially. The steam-engine changes fuel into heat and mechani-
cal motion, but there the resemblance ends. Muscle changes
its food, or fuel, not directly either into heat or motion, but into
itself ; yet as a machine it is more effective than the steam-
engine, for more work and less heat are the outcome of its
activity than is the case with the steam-engine.
THE PHYSIOLOGY OF NERVE.
Muscle and nerve are constantly associated functionally,
and have so much in common that it becomes desirable to study
them together. Much that has been established for muscle
196
COMPARATIVE PHYSIOLOGY.
holds equally well for nerve ; and the latter, though apparently
wholly different in structure at first sight, is really not so.
Nerve has its protoplasmic part (axis-cylinder), which is the
essential structure, its protective sheaths, and its nuclei (nerve-
corpuscles).
As already indicated, a nerve possesses irritability.
It is found that when the constant (polarizing) current is
passing from above downward — that is, when the cathode
(negative-pole) is on the side toward the muscle — the irritability
of the nerve is increased, and the reverse when the opposite
conditions prevail.
This altered condition is known as electrotonus.
It has been found as the result of many experiments that
profound modifications of the irritability of a nerve do take
place during the passage of a constant current. These are
diagrammatically represented in Fig. 181.
Fig. 181. — Diagrammatic representation of variations in electrotonus according to
strength of current employed (after Prliiger). nn', a section of nerve; a, anode
(+ pole); k, cathode (—pole). Curves above the horizontal denote cateleetroto-
nus; below, the opposite.
Briefly stated, they are these : 1. The nature of the change
depends on the direction of the polarizing (constant) current ;
hence, if the current is descending, there is an increase of irri-
tability (catelectrotonus) in the portion of the nerve nearest the
muscle, and vice versa. 2. The extent of the change of irrita-
bility is dependent on the strength of the polarizing current.
. 3. This change is most marked close to the electrodes, spreads
to a considerable extent beyond this point without the elec-
trodes (extra-polar regions), and also exists within the region
of contact of the electrodes (intra-po]ar regions). 4. It follows
that there must be a point at which it is not experienced (indif-
ferent point or neutral point).
Now, it is possible to understand why a sudden change in
THE STUDY OF MUSCLE PHYSIOLOGY. 197
the current should cause a muscular contraction. An equally
sudden alteration, a profound molecular effect, has been caused,
and this we must believe essential to the causation of a muscu-
lar contraction through the influence of a nerve.
To use an illustration which may serve a good purpose if
not taken too literally, it is a well-known experience that one
sitting in a room in which a clock is ticking soon fails to no-
tice this regular sound : but should the clock stop suddenly or
as suddenly commence to tick very rapidly, the attention is
aroused, while a very gradual slowing to cessation or the re-
verse would have escaped notice. The explanation of such
facts takes us down to the very foundations of biology ; but
just now we wish only to elucidate by our own experience
how it is possible to conceive of a muscle being stimulated
by the molecular movements of nerve, or rather a change in
these.
There are important practical aspects to this question. One
may understand why it is that electricity proves so ready a
stimulus, and is so valuable a therapeutic agent. It seems, in
fact, as will be learned later, to be capable of taking the place
to some extent of that constant nerve influence which we be-
lieve is being exerted in the higher animals toward the mainte-
nance of the regularity of their cell-life (metabolism).
Pathological and Clinical.— It is believed that in the nerves
of a living animal body, the electrotonic condition can be in-
duced as in an isolated piece of nerve. Hence, the value of
the constant current in diminishing nerve irritability in neu-
ralgia and allied conditions. Apparatus of great nicety of con-
struction and capable of generating, accurately measuring, and
conveniently applying electrical currents of different kinds, now
adds to the resources of the practitioner. But we are probably
as yet only on the threshold of electro-therapeutics.
Electrical Organs. — Electrical properties can be manifested
by a large number of fishes ; and the subject is of special theo-
retical interest. It is now established that the development of
electrical organs points to their being specially modified mus-
cles— tissues, in fact, in which the contractile substance has disap-
peared and the nervous elements become predominant and
peculiar. No work is done, but the whole of the chemical
energy is represented by electi*icity. Functionally an electric
organ (which usually is some form of cell, on the wTalls of
which nerves are distributed, inclosing a gelatinous substance,
198
COMPARATIVE PHYSIOLOGY.
the whole being very suggestive of a galvanic battery) closely
resembles a muscle-nerve preparation or its equivalent in the
normal body. The electric
organs experience fatigue ;
have a latent period ; their dis-
charge is tetanic (interrupted) ;
is excited by mechanical, ther-
mal, or electrical stimuli ; and
the effectiveness of the organs
is heightened by elevation of
temperature, and the reverse
by cooling, etc.
MUSCULAR WORK.
V
%
^
m
Fig. 182.— Tlie electric-fish torpedo, dissect-
ed to show electric apparatus (Huxley).
b, branchiae; c, brain; e, electric organ;
ff, cranium; me, spinal cord; //,, nerves
to pectoral fins ; nl, nervi laterales ;
np, branches of pneumpgastric nerves
to electric organs; o, eye.
If during a given period
one of two persons raises a
weight through the same
height but twice as frequent-
ly as the other, it is plain that
he does twice the work ; from
such a case we may deduce the
rule for calculating work, viz.,
to multiply the weight and
height together.
The effectiveness of a given
muscle must, of course, depend
on the degree to which it shortens, which is from one half to
three fifths of its length; and the number of fibers it contains
— i. e., upon its length and the area of its cross-section, taking
into account in connection with the first factor the arrangement
of the fibers ; those muscles in which the fibers run longitudinal-
ly being capable of the greatest total shortening.
There is, as shown by actual experimental trial, a relation
between the work done and the load to be lifted. With double
the weight tbe contraction may be as great as at first, or even
greater ; but a limit is soon reached beyond which contraction
is impossible. This principle may be stated thus: The contrac-
tion is a function of the stimulus, and is illustrated by the
diagram below (Fig. 183).
It has been shown experimentally that the chemical inter-
changes in a muscle, acting against a considerable resistance,
THE STUDY OF MUSCLE PHYSIOLOGY.
199
are increased — i. e., the metabolism and the working tension
are related.
These experimental facts harmonize with our experience of
a sense of satisfaction and effectiveness in the use of the muscles
"i — r
T~l-T
10
£0 30 40
45
50 55
60
65 70
80 90 100
Fig. 183.— Diagram of muscular contractions with same stimulus and increasing
weights. The numbers represent grammes (McKendrick).
when weights are held in the hands ; and it must be a matter
of practical importance that each person should, in taking sys-
tematic exercise, keep to that kind which does not either over-
weight or underweight the muscles.
CIRCUMSTANCES INFLUENCING THE CHARACTER
OF MUSCULAR AND NERVOUS ACTIVITY.
The Influence of Blood-Supply. Fatigue.— Fig. 184 shows at
a glance differences in the curves made by a contracting muscle
suffering from increasing fatigue.
ISO D V.
Fig. 184.— Curves of a muscle contraction in different stages of fatigue (after Yeo).
A, curve when muscle was fresh: B, C, D, E, each just after muscle had already-
contracted two hundred times. The alteration iu length of latent period is not
well brought out in these tracings.
Suppose that in such a case the blood had been withheld
from the muscle, and that it is now admitted, an almost im-
madiate effect is seen in the nature of the contractions ; but
even if only saline solution had been sent through the vessels of
the muscle, a similar change would have been noticeable. We
may fairly conclude that the blood and saline removed some-
thing which had been exercising a depressing effect on the
vitality of the muscle. In a working muscle, like all living
tissues, there are products of vital action (metabolism) that are
poisonous. We have already learned tbat a working muscle
generates an excess of carbonic anhydride, and something which
gives it an acid reaction ; and that it uses up oxygen as well as
other matters derivable from the blood.
200 COMPARATIVE PHYSIOLOGY.
Fatigue will occur, it is well known, if the muscles are used
for an indefinitely long period, no matter how favorable the
blood-supply— another evidence that there is, in all probability,
some chemical product, the result of their own activity, depress-
ing them; and this is rendered all the more likely when it is
learned that the injection of lactic acid, to take one example,
produces effects like ordinary fatigue.
It is also a matter of common experience that exercise, while
beneficial to the whole body, the muscles included, as shown by
their enlargement under it, becomes injurious when carried to
the point of fatigue.
Why the use of the muscles is conducive to their welfare is
but a part of a larger question, Why does the use of any tissue
improve it ?
When the nerve which supplies a muscle is stimulated its
blood-vessels dilate, and it has been assumed that the same
happens when a muscle contracts normally in the body ; and
when muscular action is increased there is a corresponding
augmentation in the quantity of blood driven through the
muscles in a given period, even if there be no actual increase
in the caliber of the blood-vessels, for the heart-beat is greatly
accelerated.
But repose is as necessary as exercise for the greatest effect-
iveness of the muscles, as the experience of all, and especially
athletes, proves.
That the nervous system plays a great part in the nutrition
of muscles is evident from the fact, among countless others,
that it is not possible to use the brain to its greatest capacity
and the muscles to their fullest at the same time ; the individual
engaged in physical " training " must forego severe mental ap-
plication. Nervous energy is required for the muscles, and all
questions of blood-supply are, though important, subordinate.
But it would be premature to enter into a full discussion of this
interesting topic now.
The sense of fatigue experienced after prolonged muscular
action is complex, though there can be no doubt that the nerve-
centers must be taken into account, since any muscular work
that, from being unusual, requires closer attention and a more
direct influence of the will, is well known to be more fatigu-
ing. On the other hand, the accumulation of products of
fatigue doubtless reports itself through the local nervous mech-
anism.
THE STUDY OF MUSCLE PHYSIOLOGY. 201
Separation of Muscle from the Central Nervous System.—
When the nerve belonging to a muscle is divided, certain his-
tological changes ensue, which may be briefly described as
fatty degeneration, followed by absorption ; and when regener-
ation of the nerve-fibers takes place on apposition of the cut
ends, a more or less complete restoration of the functions of
the nerve follows, but the exact nature of the process of repair
is not yet fully agreed upon ; it seems, in fact, to vary in differ-
ent cases as to details, though it is likely that, in instances in
which there is a complete return to the normal functionally,
the axis-cylinders, at all events, are reproduced.
The degeneration downward is complete ; upward, only to
the first node of Ranvier.
Immediately after the section the irritability of the nerve is
increased, but rapidly disappears, from the center toward the
periphery (Ritter-Valli law).
In the mean time the muscle has been suffering. Its nota-
bility at first diminishes, then becomes greater than usual to
shocks from the make or break of the constant current ; but
finally all irritability is lost, and fatty degeneration and disap-
pearance of true muscular structure complete the history. It
is theoretically interesting, as well as of practical importance,
that degeneration may be delayed by the use of the constant
current, the significance of which we have already endeavored
to explain.
The Influence of Temperature,— If a decapitated frog be
placed in water of the ordinary temperature, and heat be
gradually applied, the animal does not move (proving that the
spinal cord alone is not conscious), but the muscles, when 43°
to 50° C. is reached, contract and become rigid, a condition
known as " heat-rigor.1'
There are some advantages in investigating changes in tem-
perature by the graphic method. Curves from a muscle-nerve
preparation show that elevation of temperature shortens the
latent period and the curve of contraction. Lowering the tem-
perature has an exactly opposite effect, as might be supposed,
and these changes take place in the muscles of both cold-
blooded and warm-blooded animals, though more marked in
the latter.
The modifications evident to the eye are accompanied by
others, chemical in nature, and a comparison of these shows
that the rapidity and force of the muscular contraction
202 COMPARATIVE PHYSIOLOGY.
run parallel with the rapidity and extent of the chemical
changes.
Certain drugs also modify the form of the muscle-curve very
greatly, so that it appears that the' molecular action which un-
derlies all the phenomena of muscle and nerve (for what has
been said of muscle applies also to nerve, if we substitute nerv-
ous impulse for contraction) can go on only within those nar-
row bounds wbich, one realizes more and more in the study of
physiology, are set to the activities of living things.
UNSTRIPED MUSCLE.
This form of muscular tissue is characterized by its long
latent period, its slow wave of contraction, and the prog-
ress of the contraction being in either a transverse or longi-
tudinal direction, a wave of contraction in one cell being cap-
able of setting up a corresponding wave in adjoining cells
even when no nerve-fibers are distributed to them. It is ex-
cited, though less readily, by all the kinds of stimuli that act
upon striped muscle. In the higher groups of animals this
tissue is chiefly confined to the viscera of the chest and abdo-
men, constituting in the case of some of them the greater part
of the whole organ.
The slow but powerful and rhythmical contraction of this
form of muscle adapts it well to the part such organs play in
the economy. There are variations, however, in the rapidity,
force, regularity, and other qualities of the contraction in dif-
ferent parts ; thus, it is comparatively rapid in the iris, and ex-
tremely powerful and regular in the uterus, serving to produce
that prolonged yet ' intermittent pressm^e essential under the
circumstances (expulsion of the foetus).
Comparative. — Muscular contraction is relatively sluggish
and prolonged among the invertebrates, to which, however, the
movement of the wings of insects is a marked exception, some
of them having been shown by the graphic method to vibrate
some hundreds of times in a second.
The slow movements of the snail are proverbial. As a rule,
the strength of the muscles of the invertebrates is incomparably
greater than that of vertebrates, as witness the powerful grasp
of a crab's claw or a beetle's jaws.
These facts are in harmony with the generally slow metab-
olism of most invertebrates and the lower vertebrates.
THE STUDY OF MUSCLE PHYSIOLOGY. 203
The muscles of the tortoise contract tardily but with great
power, resist fatigue well, retain their vitality under unfavor-
able conditions, and after death for a very long period (days).
Without resorting to elaborate experiments, the student may
convince himself of the truth of most of the above statements
by observing the movements of a water-snail attached to a glass
vessel ; the note made by the buzzing of an insect, and compar-
ing it with one approaching it in pitch sounded by some instru-
ment of music ; the force necessary to withdraw the foot or tail
of a tortoise ; the peristaltic movements of the intestine and
other organs in a freshly killed animal ; or the action of a bee,
wasp, or wood-boring beetle on the cork of a bottle in which
one of them may be inclosed.
•
SPECIAL CONSIDERATIONS.
In the case of weakly tuberculous animals a sharp tap on
the chest will often produce a contraction of the muscles thus
stimulated; but, in addition, a local contraction lasting some
little time, known as a tvheal or iclio-muscular contraction, fol-
lows. This phenomenon seems to be the result of a special
irritability in such muscles.
Cramp may arise under a great variety of circumstances,
but it seems to be in all cases either a complete prolonged teta-
nus, in which there is unusual muscular shortening in severe
cases, at least, or the persistence of a contraction remainder.
The great differences known to exist between individuals of
the same species in strength, endurance, fleetness, and other
particulars in which the muscles are concerned, raise numer-
ous interesting inquiries. The build of the greyhound or race-
horse suggests in itself part of the explanation on mechanical
principles, lung capacity, etc. But when it is found that one
dog, horse, deer, or man excels another of the same race in
swiftness or endurance, and there is nothing in the form to
furnish a solution, we are prompted to ask whether the muscles
may not contract more energetically, experience a shortening
of the latent period or other phase of contraction ; or whether
they produce less of waste-products or get rid of them more
rapidly. The whole subject is extremely complicated, and we
may say here that there is some evidence to show that in races
of dogs and other animals which surpass their fellows the
nerve regulating the heart and lungs (vagus) has greater power ;
204 COMPARATIVE PHYSIOLOGY.
but, leaving1 this and much more out of the account, it is likely
there are individual differences in the functional nature of the
muscle. Of equal or more importance is the energizing- influ-
ence of the nervous system, which probably under great excite-
ment (public boat-races, etc.) acts to produce in man those
supermaximal contractions which seem to leave the muscle
long the worse of its unusual action. The nerve-centers, it is
likely, suffer still more from excessive discharge of nerve-force
(as we may speak of it for the present) necessary to originate
the muscular work. Hence the importance of training in all
animals to minimize the non-effective expenditure, ascertain
the capacity possessed, learn the direction in which weaknesses
lie ; and equally important the much neglected-period of rest
before actual contests — if such are to be undertaken at ail-
so that all the activities of the body may gather head, and thus
be prepared to meet the unusual demand upon them.
The law of rhythm in organic nature is beautifully illus-
trated by the behavior of nerve and especially muscle; at least
it is more obvious in the case of muscle, at this stage of our
progress.
The regularity with which one phase succeeds another in a
single contraction ; the essentially rhythmic (vibratory) char-
acter of tetanus, fatigue and recovery ; the recurrence of in-
crease and decrease in the muscle and nerve currents — in fact,
the whole history of muscle is an admirable commentary on
the truth of the law of rhythm, into which in further detail
space will not permit us to enter.
It is a remarkable fact that the endurance of man, especially
civilized man, seems to be greater than that of any other mam-
mal. It may be hazardous to express a dogmatic opinion as to
the reason of this, but the influence of the mind over the body
is unquestionably greater in man than in any other animal ;
and, if we are correct in assigning so much importance to the
influence of the nervous system in maintaining the proper
molecular balance which is at the foundation of the highest
good of an organism, we certainly think that it is in this direc-
tion we must look for the explanation of the above-mentioned
fact, and much more that would otherwise be obscure in man's
functional life.
Functional Variations.— We have endeavored, in treating
this subject of muscle, to point out how the phenomena vary
with the animal, the kind of muscle, and the circumstances
THE STUDY OF MUSCLE PHYSIOLOGY. 205
under which they are manifested. It may be shown that every
one of the qualities which a muscle possesses varies with the
temperature, the blood-supply, the duration of its action, the
character of the stimulus, and other modifying agents. Not only
are there great variations for different groups of animals, but
lesser ones for individuals ; though the latter are made more
evident indirectly than when tested by the usual laboratory
methods ; but they must be taken account of if we would un-
derstand animals as they are. Some of these will be referred
to later.
If a muscle-cell be regarded in the aspect that we are now
emphasizing, its study will tend to impress those fundamental
biological laws, the comprehension of which is of more impor-
tance than the acquisition of any number of facts, winch, how-
ever interesting, can, when isolated, profit little.
Summary of the Physiology of Muscle and Nerve.— The
movements of a muscle are distinguished from those of other
forms of protoplasm by their marked definiteness and limit-
ation.
The contraction of a muscle-fiber (cell) results in an increase
in its short transverse diameter, and a diminution of its long
diameter, without appreciable change in its total bulk.
Muscle and nerve are not automatic, but are irritable.
Though muscle normally receives its stimulus through a nerve,
it possesses independent irritability.
Stimuli may be mechanical, chemical, thermal, electrical, and
in the case of muscle, nervous ; and to be effective they must
be applied suddenly and last for a brief but appreciable time.
Electrical stimulation, especially, is only effective when
there is a sudden change in the force or direction of the cur-
rents. This applies to both muscle and nerve.
A muscular contraction consists of three phases : the latent
period, the period of rising, and the period of falling energy, or
of contraction and relaxation.
When the phase of relaxation is minimal and that of con-
traction approaches continuity, a tetanus results. The contrac-
tions of the muscles in situ are tetanic, and are accompanied
by a low sound, evidence in itself of their vibratory character.
The prolonged contraction of a muscle leads to fatigue ;
owing in part, at least, to the accumulation of waste-products
within the muscle which depress its energies.
This is a necessary consequence of the fact that all proto-
206 COMPARATIVE PHYSIOLOGY.
plasmic activity is accompanied by chemical change, and that
some of these processes result in the formation of products
which are hurtful and are usually rapidly expelled.
Muscular contraction is accompanied by chemical changes,
in which the formation of carbon dioxide, and some substance
that causes an acid reaction to take the place of an alkaline or
neutral one. Since free oxygen is not required for the act of
contraction, but is still used up by a contracting muscle, it may
be assumed that the oxygen that plays a part in actual contrac-
tion is intra-molecular.
Chemical changes are inseparable from the vital processes
of all protoplasm, and the phenomena of muscle show that
they are constantly in operation, but exalted during ordinary
contraction and that tetanic condition which precedes and
may end in coagidation of muscle plasma and the formation of
myosin. The latter is a result of the disorganization of muscle,
and has points of resemblance to the coagulation of the blood.
The contraction of a muscle and the passage of a nervous
impulse are accompanied by electrical changes. Whether cur-
rents exist in uninjured muscle and nerve is a matter of contro-
versy. Ml physiologists agree that they exist in muscle (and
nerve) duiing functional activity.
During the passage of a constant (polarizing) current from
a battery through a nerve, it undergoes a change in its irrita-
bility and shows a variation in the electro-motive force of the
ordinary nerve-current (electrotonus). This fact is of thera-
peutic importance. The electrical phenomena of nerve are
altogether more prominent than the chemical, the reverse of
which is true of muscle. The activity of a muscle (and nerve
probably) is accompanied by the generation of heat, an exalta-
tion of which takes place during muscular contraction.
Rigor mortis causes an increase in temperature and the
chemical interchanges which accompany the other phenomena.
A muscle may also become rigid by passing into rigor caloris.
Living muscle is translucent, alkaline or neutral in reaction,
and elastic ; dead muscle, opaque, acid in reaction, and devoid
of elasticity, but firmer than living muscle, owing to coagula-
tion of the muscle-plasma. Dead nerve undergoes similar
changes.
The elasticity of muscle is restricted but perfect within its
own limits. It differs from that of inorganic bodies in that the
increments of extension are not directly proportional to the
THE STUDY OF MUSCLE PHYSIOLOGY. 207
increments of the weight. When overstretched, muscle does
not return to its original length (loss of elasticity), hence the
serious nature of sprains.
It is important to regard muscular elasticity as an expression
of vital properties.
The work done by a muscle is ascertained by multiplying
the load lifted by the height; and the capacity of an individual
muscle will vary with its length, the arrangement of its fibers,
and the area of its cross-section (i. e., the number of fibers).
The work done may be regarded as a function of the resist-
ance (load), as the contraction is also a function of the stimulus.
The sepai'ation of a muscle from its nerve by section of the lat-
ter leads to certain changes, most rapid in the nerve, which
show that the two are so related that prolonged independent
vitality of the muscle is impossible, and make it highly proba-
ble that muscle is constantly receiving some beneficial stimulus
from nerve, which is exalted and manifest when contraction
takes place.
The study of the development of the electrical cells of cer-
tain fishes shows that they are greatly modified muscles in
which contractility, etc., has been exchanged for a very decided
exaltation of electrical properties. It is likely, though not
demonstrated, that all forms of protoplasm undergo electrical
changes — that these, in fact, like chemical phenomena, are vital
constants.
The phases of the contraction of smooth muscular tissue are
all of longer duration; the contraction- wave passes in different
directions, and may spread into cells devoid of nerves, which
we think not unlikely also to be the case, though less so, for all
forms of muscle.
The smooth muscle-cell must be regarded as a more primi-
tive, less specialized, form of tissue. Variations in all the phe-
nomena of muscle with the animal and the circumstances are
clear and impressive. Finally, muscle illustrates an evolution
of structure and function, and the law of rhythm.
THE NERVOUS SYSTEM.— GENERAL CONSIDER-
ATIONS.
Since in the higher vertebrates the nervous system is domi-
nant, regulating apparently every process in the organism, it
will be well before proceeding further to treat of some of its
functions in a general way to a greater extent than we have yet
done.
Manifestly, it must be highly important that an animal shall
be able to place itself so in relation to its surroundings that it
may adapt itself to them. Prominent among these adaptations
are certain movements by which food is secured and dangers
avoided. The movements having a central origin, a peripheral
mechanism of some kind must exist so as to place the centers
in connection with the outer world. Passing by the evolution
of the nervous system for the present, it is found that in verte-
brates generally there is externally a modification of the epi-
thelial covering of the body {end-organ) in which a nerve ter-
minates, which latter may be traced to a cell or cells removed
from the surface (center), and from which in most cases other
nerves proceed.
The nervous system, we may remind the student, consists in
vertebrates of centers in which nerve-cells abound, united by
nerve-fibers and by the most delicate form of connective tissue
known, in connection with which there are incased strands
of protoplasm or nerves as outgrowths. The main centers are,
of course, aggregated in the brain and spinal cord.
It is possible to conceive of the work of a nervous system
carried on by a single cell and an afferent and efferent nerve ;
but inasmuch as such an arrangement would imply that the
central cell should act the part of both receiving and origi-
nating impulses (except it were a mere conductor, in which case
there would be no advantage whatever in the existence of a cell
at all), according to tbe principle of the physiological division
NERVOUS SYSTEM.— GENERAL CONSIDERATIONS. 209
of labor, we might expect that there would be at least two cen-
tral cells — one to receive and the other to transmit impulses —
or at least that there should be some specialization among the
central cells ; and we shall have good reason later to believe
that this has reached a surprising degree in the highest ani-
mals.
Moreover, it would be a great advantage if the termination
of the ingoing (afferent) nerve should not lie exposed on the
surface, but be protected by some form of ce)l that had also the
power to transmit to it the impressions received from without,
in a form suitable to the nature of the nerve and the needs of
the organism.
So that a complete mechanism in its simplest form would
furnish : 1. A periphei'al cell or nerve end-organ. 2. An affer-
ent or sensory nerve. 3. Two or more central cells. 4. An
efferent nerve, usually connected with — 5. A muscle or other
form of cell, the action of which may be modified by the out-
going nerve, or, as we should prefer to say, by the central
nervous cells through the efferent nerve. The advantages of
the principal cells being within and protected are obvious.
When, then, an impression made on the peripheral cell is
carried inward, there modified, and results in an outgoing nerv-
ous impulse answering to the afferent one, giving rise to a mus-
cular contraction or other effect not confined to the recipient
cells, the process is termed reflex action.
Tbe great size, the multiplicity of forms, the distinct out-
line and large nuclei of nerve-cells, suggest the probability that
they play a very important part, and such is found to be the
case. Indeed, in some sense the rest of the nervous system may
be said to exist for them.
Probably nerve-cells do sometimes act as mere conductors
of nervous impulses originating elsewhere, but such is their
lowest function. Accordingly, it is found that the nature of any
reflex action depends most of all on the behavior of the central
cells.
It can not be too well borne in mind that nerves are con-
ductors and such only. They never originate impulses.
The properties considered in the last chapter are common to
all kinds of nerves known; and though we must conceive that
there are some differences in the form of impulses, these are to
be traced, not to the nerve primarily, but to the organ in which
it ends peripherally or to the central cells.
14
210 COMPARATIVE PHYSIOLOGY.
To return to reflex action, it is found that the muscular re-
sponse to a peripheral irritation vai'ies with the point stimu-
lated, the intensity of the stimulus, etc., but is, above all, deter-
mined by the central cells.
Nerve influence may be considered as following lines of
least resistance, and there is much evidence to show that an im-
pulse having once taken a certain path, it is easier for it to pass
in this direction a second time, so that we have the foundation
of the laws of habit and a host of interesting phenomena in
this simple principle.
It is found that, in a frog deprived of its brain and sus-
pended by the under jaw, there is no movement unless some
stimulus be applied ; but if this be done under suitable condi-
tions, instructive results follow, which we now proceed to indi-
cate briefly. The experiments are of a simple character, which
any student may carry out for himself.
Experimental. — Preparing a frog by cutting off the whole
of the upper jaw and brain-case after momentary anaesthesia,
suspend the animal by the lower jaw and wait till it is perfectly
quiet. Add to water in a beaker sulphuric acid till it tastes
distinctly but not strongly sour, to be used as a stimulus. 1.
Apply a small piece of bibulous paper, moistened with the acid,
to the inner part of the thigh of the animal. The leg will be
drawn up and the paper probably removed. Remove the paper
and cleanse the spot. 2. Apply a similar piece of paper to the
middle of the abdomen ; one or both legs will probably be
drawn up, and wipe off the offending body. 3. Let the foot of
the frog hang in the liquid ; after a few moments it will be
withdrawn. 4. Repeat, holding the leg ; probably the other
leg will be drawn up. 5. Apply stronger acid to the inside of
the right thigh ; the whole frog may be convulsed, or the left
leg may be put in action after the right. Even if the stimulat-
ing paper be applied near the anus, it will be removed by the
hind-legs. 6. Beneath the skin Df the back, (posterior lymph-
sac) inject a few drops of liquor strychnia? of the pharama-
copocia; after a few minutes apply the same sort of stimulus to
the thigh as before. The effects follow more quickly and are
much more marked-- the animal, it may be, passing into a gen-
eral tetanic spasm.
These experiments may be varied, but suffice to establish the
following conclusions : 1. The stimulus is not immediately
effective, but requires to act for a certain variable period, de-
NERVOUS SYSTEM.— GENERAL CONSIDERATIONS. 211
6ENSORY CENTRE
INHIBITORY CENTRE
C-^V SENSORY CELL AND
AFFERENT NERVE
Fig. 187. — Diagram intended to illustrate nervous mechanism of— 1. automatism; 2,
reflex action; and 3. how nervous impulses in the latter case may pass into the
higher parts of brain and become part of consciousness, or be wholly inhibited.
A reflex or automatic center may, for the sake of simplicity, be reduced to a sin-
gle cell, as above on the left.
pending1 chiefly on the condition of the central nervous sys-
tem. 2. The movements of the muscles harmonize (are co-ordi-
nated), and tend to accomplish some end — are purposive. If
the nerve alone and not the skin be stimulated, there may be a
spasm only and not adaptive movement. 3. Nervous impulses,
when very abundant, niay pass along unaccustomed or less ac-
customed paths (experiments 4 and 5). This is sometimes spoken
of as the radiation of nervous impulses.
The sixth experiment is very important, for it shows that
the result varies far more with the condition of the nervous
centers (cells) than the stimulus, the part excited, or any other
factor.
Automatism. — But, seeing that these central cells have such
independence and controlling power, the question arises. Are
212 COMPARATIVE PHYSIOLOGY.
these, or are there any such cells, capable of originating im-
pulses in nerves wholly independent of any stimulus from
without ? In other words, have the nerve-centers any true
automatism ? Apparently this quality is manifested by uni-
cellular organisms of the rank of Amceba. Has it been lost, or
has it become a special characteristic developed to a high degree
in nerve-cells ?
We shall present the facts and the opinions based on them
as held by the majority of physiologists, reserving our own
criticisms for another occasion : 1. The medulla oblongata is
supposed to be the seat of numerous small groups of cells, to a
large extent independent of each other, that are constantly
sending out nervous impulses which, proceeding to certain sets
of muscles, maintain them in rhythmical action. One of the
best known of these centers is the respiratory. 2. The poste-
rior lymph hearts of the frog are supplied by nerves (tenth
pair), which are connected, of course, with the spinal cord.
When these nerves are cut, the hearts for a time cease to beat,
but later resume their action. 3. The heart beats after all its
nerves are cut, and it is removed from the body, for many hours,
in cold-blooded animals. 4. The contractions of the intestine
take place in the absence of food, and in an isolated piece of
the gut. The intestine, it will be remembered, is abundantly
supplied with nerve-elements. 5. In a portion of the ureters,
from which it is believed nerve-cells are absent, rhythmical ac-
tion takes place.
Conclusions. — 1. Whether the action of the respiratory and
similar centers could continue in the absence of all stimuli can
not be considered as determined. 2. That there are regular
rhythmical discharges from the spinal nerve-cells along the
nerves to the lymph hearts seems also doubtfbl. 3. Later in-
vestigations render the automaticity of the heart more uncer-
tain than ever, so that the result stated above (3) must not be
interpreted too rigidly.
Similar doubts hang about the other cases of apparent au-
tomatism.
As regards the various comparatively isolated collections of
cells known as ganglia, the evidence, so far as it goes, is against
their possessing either automatic or reflex action ; and new
views of their nature will be presented in due course.
Nervous Inhibition. — If the pneumogastric nerve passing
from the medulla to the heart of vertebrates be divided and the
NERVOUS SYSTEM.— GENERAL CONSIDERATIONS. 213
lower (peripheral) end stimulated, a decided change in the ac-
tion of the heart follows, which may be in the direction of"
weakening- or slowing, or positive arrest of its action.
Assuming, for the present, that the cells (center) of the me-
dulla have the power to bring about the same result, it is seen
that such nervous influence is preventive or inhibitory of the
normal cardiac beat, so that the vagus is termed an inhibitory
nerve. Such inhibition plays a very important part in the
economy of the higher animals, as will become rnore and more
evident as we proceed. The nature of the influences that pro-
duce such remarkable results will be discussed when we treat
of the heart.
An illustration will probably serve in the mean time to make
the meaning of what has been presented in this chapter more
clear and readily grasped.
In the management of railroads a very great variety of com-
plicated results are brought about, owing to system and orderly
arrangement, by which the wishes of the chief manager are
carried out.
Telegraphing is of necessity extensively employed. Sup-
pose a message to be conveyed from one office to another, this
may (1) simply pass through an intermediate office, without
special cognizance from the operator in charge ; (2) the operator
may receive and transmit it unaltered ; (3) he may be required
to send a message that shall vary from the one he receives in a
greater or less degree ; or (4) he may arrest the command alto-
gether, owing to the facts which he alone knows and upon
which he is empowered always to act according to his best dis-
cretion.
In the first instance, we have an analogy with the passage
of a nervous impulse through central fibers, or, at all events,
unaffected by cells ; in the second, the resemblance is to cells
acting as conductors merely ; in the third, to the usual behavior
of the cells in reflex action; and, in the fourth, we have an in-
stance of inhibition. The latter may also be rendered clear by
the case of a horse and its rider. The horse is controlled by the
rider, who may be compared to the center, through the reins
answering to the nerves, though it is not possible for either rider
or reins to originate the movements of the animal, except as
they 'may be stimuli, which latter are only effective when there
are suitable conditions— when, in fact, the subject is irritable in
the physiological sense.
THE CIRCULATION OF THE BLOOD.
Every tissue, every cell, requiring constant nourishment,
some means must necessarily have been provided for the con-
veyance of the blood to all parts of the organism. We now
enter upon the consideration of the mechanisms by which this
is accomplished and the method of their regulation.
Let us consider possible mechanisms, and then inquire into
their defects and the extent to which they are found embodied
in nature.
That there must be a central pump of some kind is evident.
Assume that it is one-chambered, and with an outflow-pipe
which is continued to form an inflow-pipe. This might be pro-
vided with valves at the openings, by which energy would be
saved by the prevention of regurgitation. In such a system
things must go from bad to worse, as the tissues, by constantly
using up the prepared material of the blood, and adding to it
their waste products, would effect their own gradual starvation
and poisoning.
It might be conceived, however, that waste at all events was
got rid of by the blood being conducted through some elimi-
nating organs ; and assume that one such at least is set aside
for respiratory work. If the blood in its course anywhere
passed through such organs, the end would be attained in some
degree ; but if the division of labor were considerable, we
should suppose that, gaseous interchange being so very impor-
tant as we bave been led to see from the study of the chapters
on general biology, and on muscle, organs to accomplish this
work might receive the blood in due course and return it to the
central pump in a condition eminently fit from a respiratory
point of view.
Such, however, would necessarily be associated with a more
complicated pump ; and, if this were so constructed as to pre-
vent the mixture of blood of different degrees of functional
value, higher ends would be attained.
THE CIRCULATION OP THE BLOOD. 215
Turning to the channels themselves in which the blood
flows, a little consideration will convince one that rigid tubes
are wholly unfit for the purpose. Somewhere in the course of
the circulation the blood must flow sufficiently slowly, and
through vessels thin enough to permit of that interchange be-
tween the blood and the tissues, through the medium of the
lymph, which is essential from every point of view. The main
vessels must have a strength sufficient to resist the force with
which the blood is driven into them.
Now, it is possible to conceive of this being accomplished
with an intermittent flow ; but manifestly it would be a great
advantage, from a nutritive aspect, that the flow and therefore
the supply of tissue pabulum be constant. With a pump regu-
larly intermittent in action, provided with valves, elastic tubes
having a resistance in them somewhere sufficient to keep them
constantly over-distended, and a collection of small vessels with
walls of extreme thinness, in which the blood-current is greatly
slackened, a steady blood-flow would be maintained, as the
student may readily convince himself, by a few experiments of
a very simple kind :
1. To show the difference between rigid tubes and elastic
ones, let a piece of glass-rod, drawn out at one end to a small
diameter, have attached to the other end a Higginson's (two-
bulb) syringe, communicating with a vessel containing water.
Every time the bulb is squeezed, water flows from the end of
the glass rod, but the outflow is perfectly intermittent.
2. On the other hand, with a long elastic tube of India-rub-
ber, ending in a piece of glass rod drawn out to a point as be-
fore, if the action of the pump (bulb) be rapid the outflow will
be continuous. An apparatus that every practitioner of medi-
cine requires to use answers perhaps still better to illustrate
these and other principles of the circulation, such as the pulse,
the influence of the force and frequency of the heart-beat on the
blood-pressure, etc. We refer to a two-bulb atomizer, the bulb
nearer the outflow serving to maintain a constant air-pressure.
We may now examine the most perfect form of heart
known, that of the mammal, in order to ascertain how far it
and its adjunct tubes answer to a priori expectations.
The Mammalian Heart.— In order that the student may gain
a correct and thorough knowledge of the anatomy of the heart
and the, workings of its various parts, we recommend him to
pursue some such course as the following :
216
COMPARATIVE PHYSIOLOGY.
1. To consult a number of plates, such as are usually fur-
nished in works on anatomy, in order to ascertain in a general
way the relations of the heart to other organs, and to the chest
wall, as well as to become familiar with its own structure.
2. To supplement
this with reading the
anatomical descrip-
tions, without too great
attention to details at
first, but with the ob-
ject of getting his ideas
clear so far as they go.
3. Then, with plates
and descriptions before
him, to examine sever-
al dead specimens of
the heart of the sheep,
ox, pig, or other mam-
mal, first somewhat
generally, then syste-
matically, with the
purpose of getting a
more exact knowledge
of the various struct-
ures and their anatom-
ical as well as physi-
ological relations.
We would not have
the student confine his
attention to any single
form of heart, for each
shows some one struct-
ure better than the
others ; and the addi-
tional advantages of
comparison are very
great. The heart of
the ox, from its size,
is excellent for the study of valvular action, and the framework
with which the muscles, valves, and vessels are connected ;
while the heart of the pig (and dog) resemble the human organ
more closely than most others that can be obtained.
I'n;. 186.— The left auricle and ventricle opened and
pari of their walls removed to show their cavities
(Allen Thomson). 1, right pulmonary vein cut
short; V, cavity of left auricle; 3, thick wall of
left ventricle ; 4, portion of the same with pap-
illary muscle attached ; 5, 5', the other papillary
muscles; 0, one segment of the mitral valve; 7,
in aorta is placed over the semilunar valves.
THE CIRCULATION OF TI1E BLOOD.
217
It will be found very helpful to perform some of the dissec-
tions under water, and by the use of this or some other fluid
the action of the valves may be learned as it can in no other
way. By a little manipulation the heart may be so held that
water may be poured into the orifices, prepared by a removal
of a portion of the blood-vessels or the auricles, when the valves
may be seen closing together, and thus revealing their action in
a way which no verbal or pictorial representation can do at all
adequately.
A heart thoroughly boiled and allowed to get cold shows, on
being pulled somewhat apart, the course, attachment, and other
■It T^m.v.1
JRAV
Fio. 187. -View of the orifices of the heart from below, the whole of the ventricles
having been cut away (after Huxley). JRAV, right auriculo-ventricular orifice,
surrounded bv the three flaps, t. v. 1, t. v. 2. t. v. 3, of the tricuspid valve, which are
stretched by weights attached to the chorda tendinece. LAV. left auriculo-ven-
tricular orifice, etc. PA. orifice of the pulmonary artery, the semilunar valves
represented as having met and closed together. A 0, orifice of the aorta.
features of the fibers very well, as also the skeleton of the organ,
which may be readily separated.
When this has all been done, the half is not yet accom-
plished. A visit to an abattoir will now repay amply for the
time spent. Animals are there killed and eviscerated so rapidly
that an observer may not only gain a good practical acquaint-
ance with the relations of the heart to other parts, but may
often see the organ still living and exemplifying that action
218
COMPARATIVE PHYSIOLOGY.
peculiar to it as it gradually approaches quiescence and death
— a matter of the utmost importance.
If the student will then compare what he has learned of the
mammalian heart in this way with the behavior of the heart
of a frog-, snake, fish, turtle, or other animal that may be killed
after brief ether narcosis, without cessation of the heart's ac-
tion, he will have a broader basis for his cardiac physiology
than is usual ; and we think we may promise the medical stu-
dent, who will in this and other ways that may occur to him
supplement the usual work on the human cadaver, a pleasure
and profit in the study of heart-disease which come in no
other way.
With the view of assisting the observation of the student
as regards the heart of the mammal, we would call special atten
tion to the following points among others : Its method of sus-
pension, chiefly by its great vessels ; the strong fibrous frame-
work for the attachment of valves, vessels, and muscle-fibers;
the great complexity of the arrangement of the latter; the
various lengths, mode of attachment, and the strength of the
PA
Fig. 188.— Orifices of the heart seen from above, after the auricles and great vessels
had been cut awav (after Huxley). PA . pulmonary artery with its semilunar valves.
Ao, aorta in a similar condition. RAV, right auriculo-ventricular orifice, with
m. v. 1 and 2 flaps of mitral valve: b. style passed into coronary vein. On the left
part of LA Fthe section of the auricle is carried through the auricular appendage,
hence the toothed appearance due to the portions in relief cut.across.
inelastic chordae tendinese; the papillary muscles, which doubt-
less act at the moment the valves flap back, thus preventing
THE CIRCULATION OF THE BLOOD. 219
the latter being carried too far toward the auricles, the pocket-
ing action of the semilunar valves with their strong margin
and meeting nodules {corpora Arantii) ; the relative thickness
of auricles and ventricles, and the much greater thickness of
the walls of the left than of the right ventricle— differences
which are related to the work these parts perform.
The latter may be well seen by making transverse sections
of the heart of an animal, especially one that has been bled to
death, which specimen also shows how the contraction of the
heart obliterates the ventricular cavity.
It will also be well worth while to follow up the course
of the coronary arteries, noting especially their point of
origin.
The examination of the valves of the smaller hearts of cold-
blooded animals is a matter of greater difficulty and is facili-
tated by dissection under water with the help of a lens or dis-
secting microscope ; but even without these instruments much
may be learned, and certainly that the valves are relatively to
those of the mammalian heart imperfectly developed, will be-
come very clear.
CIRCULATION OF THE BLOOD IN THE MAMMAL.
It is highly important and quite possible in studying the
circulation to form a series of mental pictures of what is trans-
piring. It will be borne in mind that there is a set of elastic
tubes of relatively thick walls, standing open when cut across,
dividing into smaller and smaller branches, and finally ending
in vessels of more than cobweb fineness, and opening out into
others, that become larger and larger and fewer and fewer, till
they are gathered up into two of great size which form the right
auricle. The larger pipes consist everywhei'e of elastic tissue
proper, muscular tissue (itself elastic), fibrous tissue, and a flat
epithelial lining, so smooth that the friction therefrom must be
minimal as the blood flows over it.
The return tubes or veins are like the arteries, but so thin
that their walls fall together when cut across. They are differ-
ent from all the other blood-tubes in that they possess valves
opening toward the heart throughout their course. The veins
are at least twice as numerous as the arteries, and their capacity
many times greater. The small vessels or capillaries are so
abundant and wide-spread that, as is well known, the smallest
220
COMPARATIVE PHYSIOLOGY.
cut anywhere gives rise to a
flow of blood, owing to sec-
tion of some of these tubes,
which, it will be remembered,
are not visible to the unaided
eye. It is estimated that their
united area is several hun-
dred (500 to 800) times that of
the arteries.
If we suppose the epithe-
lial lining pushed out of a
small artery we have, so far
as structure alone goes, a
good idea of a capillary — i. e.,
its walls are but one cell
thick, and these cells though
loug are extremely thin, so
that it is quite easy to under-
stand how it is that the amoe-
boid corpuscles can, under
certain circumstances, push
tbeir way through its proba-
bly semi-fluid walls.
From what has been said,
it will be seen that the whole
collection of vascular tubes
may be compared to two inverted funnels or cones with the
Fig. 189.— Various layers of the walls of a
small artery (Landois). e, endothelium;
i. e, internal elastic lamina; c. m, circu-
lar muscular fibers of the middle coat;
c. t, connective tissue of the outer coat,
or T. adventitia.
Fig. 190.
Fio. 191.
Fig. 190.— Vein with valves lying open (Dalton).
In. 191.— Vein with valves closed, the blood passing on by a lateral branch below
(Dalton).
THE CIRCULATION OF THE BLOOD.
221
wIfi iqo _CaDillarv blood-vessels (Landois). The cement substance between the en-
dothelium h'^Len rendered dark by silver nitrate, and the nuclei made prominent
by staining.
smaller end toward the heart aud the widest portions repre-
senting the capillaries.
Pig. 193—Diagram to illustrate the relative proportions of the ^egate sectional we.;
of the different parts of the vascular system (after \eo). A, aorta, C, capillars .
V, veins.
222
COMPARATIVE PHYSIOLOGY,
THE ACTION OF THE MAMMALIAN HEART.
What takes place may be thus very briefly stated : The
right auricle contracting- squeezes the blood through the au-
ricular-ventricular opening into the right ventricle, never quite
Superior Vena
Cava.
Inferior Vena
Cava.
Capillaries of the
Head, etc.
Pulmonary Ca-
pillaries.
Capillaries of
Trunk and
Lower Ex-
tremities.
Fk;. 194.— Diagram of the circulation. The arrows indicate the course of the blood.
Though the pulmonary, the lower and the upper parts of the systemic circulation
are represented so as to show the distinctness of each, it will be also apparent that
they are not independent. Relative size of different parts of the system is only
very generally indicated.
emptying itself probably; immediately after the right ventricle
contracts, by which its valves are brought into sudden tension
and opposition, thus preventing reflux into the auricle ; while
the blood within it takes the path of least resistance, and the
THE CIRCULATION OP THE BLOOD. 223
only one open to it into the pulmonary artery, and by its
branches is conveyed to the capillaries of the lungs, from which
it is returned freed from much of its carbonic anhydride and
replenished with oxygen, to the left auricle, whence it proceeds
in a similar manner into the great arterial main, the aorta, for
general distribution throughout the smaller arteries and the
capillaries to the most remote as well as the nearest parts, from
which it is gathered up and returned laden with many impuri-
ties, and robbed of a large proportion of its useful matters, to
the right side of the heart.
It will be remembered that corresponding subdivisions of
each side of the heart act simultaneously, and that any decided
departure from this harmony of rhythm would lead to serious
disturbance.
THE VELOCITY OF THE BLOOD AND BLOOD-PRESSURE.
If the relative capacity and arrangement of the various parts
of the circulatory system be as has been represented, it follows
that we may predict with some confidence, apart from experi-
ment, what the speed of the flow and the vascular tension must
be in different parts of the course of the circulation.
We should suppose that, in the nature of the case, the veloc-
ity would be greatest in the large arteries, gradually diminish
to the capillaries, in which it would be much the slowest and,
getting by degrees faster, would reach a speed in the largest
veins approaching that of the corresponding arteries.
The methods of determining the velocity of the blood-stream
have not entirely surmounted the difficulties, but they do give
results in harmony with the above-noted anticipations.
The area of the great aortic trunk being so much less than
that of the capillaries, the flow in that vessel we should expect
to be very much swifter than in the arterioles or the capillaries.
Moreover there must be a great difference in the velocity during
cardiac systole and diastole, and according as the beat of the
heart is forcible or otherwise. But apart from these more ob-
vious differences, there are variations depending on complex
changes in the peripheral circulation, owing to the frequent
variations in the diameter of the arterioles in different parts,
as well as differences in the resistance offered by the capillaries,
the causes of which are but ill understood, though less obscure,
we think, than they are often represented to be. Since for the
224 COMPARATIVE PHYSIOLOGY.
maintenance of the circulation, the quantity of blood entering
and leaving the heart must be equal, in consequence of the sec-
tional area of the great veins that enter the heart being greater
than that of the aorta, it follows that the venous flow even at its
quickest is necessarily slower than the arterial.
Comparative. — There must be great variations in velocity in
different animals, as such measurements as have been made
demonstrate. Thus, in the carotid of the horse, the speed of
the blood-current is calculated as about 306 mm., in the dog at
from 205 to 357 mm. These results can not be considered as
more than fair approximations.
Highly important is it to note that the rate of flow in the
capillaries of all animals is very slow indeed, not being as much
as 1 mm. in a second in the larger mammals. The time occupied
by the circulation is also, of course, variable, being as a rule
shorter the smaller the animals. As the result of a number of
calculations, though by methods that are more or less faulty,
the following law may be laid down as meeting approximately
the facts so far as warm-blooded animals are concerned.
The circulation is effected by 27 beart-beats ; thus for a man
with a pulse of 81, the time occupied in the completion of the
course of the blood from and to the heart would be f| = 3 ; i. e.,
the circulation is completed three times in one minute, or its
period is twenty seconds ; and it is to be well borne in mind
that by far the greater part of this time is occupied in traversing
the capillaries.
THE CIRCULATION UNDER THE MICROSCOPE.
There are few pictures more instructive and impressive than
a view of the circulation of the blood under the microscope.
It is well to have similar preparations, one under a low power
and another under a magnification of 300 to 500 diameters.
With the former a view of arterioles, veins, and capillaries may
be obtained at once. Many different parts of animals may be
used, as the web of the frog's foot, its tongue, lung, or mesen-
tery ; the gill or tail of a small fish, tadpole, etc.
The relative size of the vessels ; the speed of the blood flow;
the greater velocity of the central part of the stream ; the aggre-
gation of colorless corpuscles at the sides of the vessels, and the
occasional passage of one through a capillary wall, when the
exposure has lasted some time; the crowding of the red cells;
THE CIRCULATION OF THE BLOOD.
225
their plasticity ; the small size of some of the capillaries, barely
allowing the corpuscles to be squeezed through; the changes in
the velocity of the current, especially in the capillaries ; its pos-
sible arrest or retrocession ; the velocity in one so much greater
than in its neighbor, without very obvious cause — all this and
Fig. 195. — Portion of the web of a frog's foot as seen under a low magnifying power,
showing the blood-vessels, and in one corner the pigment-spots (after Huxley), a,
small arteries (arterioles); v, small veins. The smaller vessels are the capillaries.
The course of the blood is indicated by arrows.
much more forms, as we have said, a remarkable lesson for the
thinking student. This, like all microscopic views, especially
if motion is represented, has its fallacies. It is to be remem-
bered that the movements are all magnified, or else one is apt
to suppose the capillary circulation, extremely rapid, whereas
it is like that of the most sluggish part of a stream, and very
irregular.
15
226
COMPARATIVE PHYSIOLOGY.
Fig. 196.— Circulation in the web of a frog's foot (Wagner). V, venous trunk com-
posed of the three principal branches (v. v, v) covered with a plexus of smaller ves-
sels. The whole is dotted over with pigment masses.
THE CHARACTERS OF THE BLOOD-FLOW.
If an artery be opened, the blood is seen to flow from it in
a constant stream, with periodic exaggerations, which, it is
found, answer to the heart-beats ; in the case of veins and
capillaries the flow is also constant, but shows none of the
spurting of the arterial stream, nor has the cardiac beat appar-
ently an equal modifying effect upon it.
We have already explained why the flow should be constant,
though it would be well to be clearer as to the peripheral re-
sistance. The amount of friction from linings so smooth as
those of the blood-vessels can not be considerable. Whence,
then, arises that friction which keeps the arterial vessels always
distended by its backward influence ? The microscopic study
of the circulation helps to answer this question. The plas-
ticity of the corpuscles and of the vessel walls themselves must
be taken into account, in consequence of which a dragging
influence is exerted whenever the corpuscles touch the wall,
which must constantly happen with vast numbers of them in
the smallest vessels and especially in the capillaries. The
arrangement of capillaries into a mesh-work, must also, in
THE CIRCULATION OF THE BLOOD. 227
consequence of so many angles, be a source of much fric-
tion.
The action of the corpuscles on one another may be com-
pared to a crowd of people hurrying along a narrow passage —
the obstruction comes from interaction of a variety of forces,
owing to the crowd itself rather than the nature of the thor-
oughfare. We must set down a great deal to the influence of
the corpuscles on one another, as they are carried along accord-
ing to mechanical principles ; but, as we shall see later, other
and more subtle factors play a part in the capillary circulation.
Owing to the peripheral resistance and the pumping force of
the heart, the arteries become distended, so that, during cardiac
diastole, their recoil, owing to the closure of the semilunar
valves, forces on the blood in a steady stream. It follows, then,
that the main force of the heart is spent in distending the
arteries, and that the immediate propelling force of the circu-
lation is the elasticity of the arteries hi which the heart stores
up the energy of its systole for the moment.
BLOOD-PRESSURE.
Keeping in mind our schematic representation of the circu-
lation, we should expect that the blood must exercise a certain
pressure everywhere throughout the vascular system ; that this
blood-pressure would be highest in the heart itself ; considera-
ble in the whole arterial system, though gradually diminishing
toward the capillaries, in which it would be feeble ; lower still
in the smaller veins ; and at its minimum where the great veins
enter the heart. Actual experiments confirm the truth of these
views ; and, as the subject is one of considerable importance, we
shall direct attention to the methods of estimating and record-
ing an animal's blood-pressure.
First of all, the well-known fact that, when an artery is cut,
the issuing stream spurts a certain distance, as when a water-
main, fed from an elevated reservoir, bursts, or a hjTdrant is
opened, is itself a proof of the existence of blood-pressure, and
is a crude measure of the amount of the pressure.
One of the simplest and most impressive ways of demon-
strating blood-pressure is to connect the carotid, femoral, or
other large artery of an animal by means of a small glass tube
(drawn out in a peculiar manner to favor insertion and reten-
tion by ligature in the vessel), known as a cannula, by rubber
228
COMPARATIVE PHYSIOLOGY.
Fig. 197.
THE CIRCULATION OB" THE BLOOD. 229
Fi«. 19?.— Apparatus used in making a blood-pressure experiment (after Foster) //. b.
pressure-bottle, elevated so as to raise the pressure several inches of mercury, as
seen in the manometer (m) below. It contains a saturated solution of sodium car-
bonate; r. t, rubber tube connecting the pb with the leaden tube: /. t, tube made
of lead, so as to be pliable, yet have rigid walls; t«. c, a stop-cock, the top of which
is removable, to allow escape of bubbles of air; p, the pen, writing on the roll of
paper, r. The former floats on the mercury; m, the manometer, the shaded por-
tion of the bent tube denoting the mercury, the rest is tilled with a fluid unfavor-
able to the coagulation of the Dlood, and derived from the pressure-bottle; ca, the
carotid, in which is placed the cannula, and below the latter a forceps, which may
be removed when the blood-pressure is to be actually measured. The registration
of the height, variation, etc., of blood-pressure, is best made on a continuous roll
of paper, as seen in Fig. 198.
tubing, with a long glass rod of bore approaching that of the
artery opened, into which the blood is allowed to flow through
the above-mentioned connections, while it is maintained in a
vertical position.
To prevent the rapid coagulation of the blood in such ex-
periments, it is customary to fill the cannula and other tubes
to a certain extent, at least, with a solution of some salt that
tends to retard coagulation, such as sodium carbonate or bicar-
bonate, magnesium sulphate, etc. If other connections are
made in a similar way with smaller arteries and veins, it may
be seen that the height of the respective columns representing
the blood-pressure, varies in each and in accordance with ex-
pectations.
While all the essential facts of blood-pressure and many
others may be illustrated by the above simple methods, it is inad-
equate when exact measurements are to be made or the results
to be recorded for permanent preservation ; hence apparatus of
a somewhat elaborate kind has been devised to accomplish these
. purposes.
The graphic methods are substantially those already ex-
plained in connection with the physiology of muscle ; but, since
it is often desirable to maintain blood-pressure experiments
for a considerable time, instead of a single cylinder, a series so
connected as to provide a practically endless roll of paper (Fig.
198) is employed.
When, in the sort of experiments referred to above, the
height of the fluid used in the glass tube to prevent coagula-
tion just suffices to prevent outflow from the artery into the
connections, we have, of course, in this a measure of the blood-
pressure; however, it is convenient in most instances to use
mercury, contained in a glass tube bent in the form of a U. for
a measure, as shown in the subjoined illustration. It is also
desirable, in order to prevent outflow of the blood into the
apparatus, to get up a pressure in the U-tube or manometer as
230
COMPARATIVE PHYSIOLOGY.
near as may be equal to that of the animal to be employed in
the experiment. This may be effected in a variety of ways, one
of the most convenient of which is by means of a vessel con-
taining some saturated sodium carbonate or similar solution in
connection with the manometer.
It is important that the pressure should express itself as
directly and truthfully on the mercury of the manometer as
possible, hence the employment of a tube with rigid walls, yet
capable of being bent readily in different directions for the sake
of convenience.
Mercury, on account of its inertia, is not free from objec-
tion ; and when very delicate variations in the blood-pressure —
Pia. 198.— Large kymograph, with continuous roll of paper (Foster). The clock-work
machinery unrolls the paper from the roll C, carries it smoothly over the cylinder
B, and then winds it up into the roll A. Two electro-magnetic markers are seen
in position recording intervals of time on the moving roll of paper. A manometer
may !><• fixed in any convenient position.
e. g., feeble pulse-beats — are to be indicated, it fails to express
them, in which case other fluids may be employed.
It will be noted that when an ordinary cannula is used,
inserted as it is lengthwise into the blood-vessel, the pressure
recorded is not that on the side of the vessel into which it is
inserted as when a —\ - piece is used, but of the vessel, of which
the one in question is a branch. The blood-pressure, in the
main arterial trunk, for example, must depend largely on the
THE CIRCULATION OF THE BLOOD. 231
force of the heart-beat ; consequently it would be expected, and
it is actually found, that the pressure varies for different ani-
mals, size having, of course, in most instances a relation to
the result. It has been estimated that in the carotid of the horse
the arterial pressure is 150 to 200 mm. of mercury, of the dog
100 to 175, of the rabbit 50 to 90. Man's blood-pressure is not
known, but is probably high, we may suppose not less than
150 to 200 mm.
After the fact that there is a certain considerable blood-
pressure, the other most important one to notice is that this
blood-pressure is constantly varying during the experiment,
and, as we shall give reason to believe, in the normal animal ;
and to these variations and their causes we shall presently turn
our attention.
THE HEART.
The heart, being one of the great centers of life, to speak
figuratively, it demands an unusually close study.
THE CARDIAC MOVEMENTS.
There is no special difficulty in ascertaining the outlines of
the heart by means of percussion on either the dead or the
living subject. Quite otherwise is it with the changes in form
which accompany cardiac action. Attempts have been made
to ascertain the alterations in position of the heart with respect
to other parts, and especially its own alterations in shape dur-
ing a systole, the chest being unopened, by the use of needles
thrust into its substance through the thoracic walls ; but the re-
sults have proved fallacious. Again, casts have been made of
the heart after death, in a condition of moderate extension, prior
to rigor mortis ; and also when contracted by a hardening fluid.
These methods, like all others as yet employed, are open to seri-
ous objections.
Following the rapidly beating heart of the mammal with
the eye produces uncertainty and confusion of mind.
It may be very confidently said that the mode of contrac-
tion of the hearts of different groups of vertebrates is variable,
though it seems highly probable that the divergences in mam-
mals are slight. The most that can be certainly affirmed of the
mammalian heart is, that during contraction of the ventricles
232 COMPARATIVE PHYSIOLOGY.
they become more conical ; that the long diameter is not appre-
ciably altered ; that the antero-posterior diameter is lengthened ;
and that the left ventricle at least turns on its own axis from
left to right. This latter may be distinctly made out by the eye
in watching the heart in the opened chest.
THE IMPULSE OF THE HEART.
When one places his hand over the region of the heart in
man and other mammals, he experiences a sense of pressure
varying with the part touched, and from moment to moment.
Instruments constructed to convey this movement to recording
Fig,, 199.— Marey's cardiac sound, which may be used to explore the chambers of the
heart (after Foster), a is made of rubber stretched over a wire framework, with
metallic supports above and below; b is a long tube.
levers also teach that certain movements of the chest wall cor-
respond with the propagation of the pulse, and therefore to the
systole of the heart. It can be recognized, whether the hand
or an instrument be used, that all parts of the chest wall over
the heart are not equally raised at the one instant. If the beat-
ing heart be held in the hand, it will be noticed that during
systole there is a sudden hardening. The relation of the apex
to the chest wall is variable for different mammals, and with
different positions of the body in man.
As a result of the investigation which this subject has re-
ceived, it may be inferred that the sudden tension of the heart,
owing to the ventricle contracting over its fluid contents,
causes in those cases in which during diastole the ventricle
lies against the chest wall, a sense of pressure beneath the
hand, which is usually accompanied by a visible movement
upward in some part of the thoracic wall, and downward in
adjacent parts.
It will not be forgotten that the heart lies in a pericardial
sac, moistened with a small quantity of albuminous fluid ; and
that by this sac the organ is tethered to the walls of the chest
by its mediastinal fastenings ; so that in receding from the
chest wall the latter may be drawn after it ; though this might
THE CIRCULATION OF THE BLOOD.
233
Cardiac
impulse.
Fig. 200.— Simultaneous tracings from the interior of the right auricle, from the inte-
rior of the right ventricle, and of the cardiac impulse in the horse (after Chauveau
and Marey). Tracings to be read from left to right, and the references above are
in the order from top to bottom. A complete cardiac cycle is included between
the thick vertical lines I and II. The thin vertical lines indicate tenths of a sec-
ond. The gradual rise of pressure within the ventricle (middle tracing) during
diastole, the sudden rise with the systole, its maintenance with oscillations for an
appreciable time, its sudden fall, etc., are all well shown. There is disagreement
as to the exact meaning of the minor curves in the larger ones.
also follow from the intercostal muscles being simply unsup-
ported when the heart recedes.
INVESTIGATION OP THE HEART-BEAT FROM
WITHIN.
By the use of apparatus, introduced within the heart of the
mammal and reporting those changes susceptible of graphic
record, certain tracings have been obtained about the details of
which there are uncertainty and disagreement, though they
seem to establish the nature of the main features of the cardiac
beat clearly enough. An interpretation of such tracings in tbe
light of our general and special knowledge warrants the follow-
ing statement.
1. Both auricular and ventricular systole are sudden, but the
latter is of very much greater duration.
2. While the chest wall feels the ventricular systole, the
auriculo-ventricular valves shield the auricle from its shock.
3. During diastole in both chambers tbe pressure rises gradu-
234 COMPARATIVE PHYSIOLOGY.
ally from, the inflow of "blood ; and the auricular contraction
produces a brief, decided, though but slight rise of pressure in
the ventricles.
4. The onset of the ventricular systole is rapid, its maximum
pressure suddenly reached, and its duration considerable.
The relations of these various events, their duration and
the corresponding movements of the chest wall, may be learned
by a study of the above tracing which the student will find
worthy of his close attention.
THE CARDIAC SOUNDS.
Two sounds, differing in pitch, duration, and intensity, may
be heard over the heart when the chest is opened and the heart
listened to by means of a stethoscope. These sounds may also
be heard, and present the same characters when the heart is
auscultated through the chest wall ; hence the cardiac impulse
can take no essential part in their production.
The sounds are thought to be fairly well represented, so far
as the human heart is concerned, by the syllables lub, dup ; the
first sound being longer, louder, lower-pitched, and '" booming "
in quality ; the second short, sharp, and high-pitched.
In the exposed heart, the first sound is heard most distinctly
over the base of the organ or a little below it ; while the sec-
ond is communicated most distinctly over the roots of the great
vessels — that is to say, both sounds are heard best over the
auriculo-ventricular and semilunar valves respectively. When
the chest Wall intervenes between the heart and the ear, it is
found that the second sound is usually heard most distinctly
over the second costal cartilage on the right ; and the first in
the fifth costal interspace where the heart's impulse is also often
most distinct. In these situations the arch of the aorta in the
one case, and the ventricular walls in the other, are close to the
situations referred to during the cardiac systole ; hence it is
inferred that, though the sounds do not originate directly be-
neath these spots, they are best propagated to the chest wall at
these points. Prior to the study of the heart in our domestic
animals the student is recommended to investigate the subject
on himself by means of a double stethoscope or on another per-
son with or without any instruments.
There are, however, individual differences, owing to a va-
riety of causes, which it is not always possible to explain fully
THE CIRCULATION OP THE BLOOD.
235
in each case, but owing doubtless in great part to variations in
anatomical relations.
The Causes of the Sounds of the Heart.— There is general
agreement in the view that the second sound is owing to the
closure of the semilunar valves of the aortic and pulmonary
vessels ; the former, owing to their greater tension in conse-
quence of the higher blood-pressure in the aorta, taking much
the larger share in the production of the sound, as may be
ascertained by listening over these vessels in the exposed heart.
When these valves are hooked back, the second sound disap-
pears, so that there can be no doubt that they bear some impor-
tant relation to the causation of the sound.
In regard to the first sound of the heart the greatest diversity
of opinion has prevailed and still continues to exist. The fol-
lowing among other views have been advocated by physiolo-
gists :
1. The first sound is caused by the tension and vibration of
the auriculo- ventricular valves.
2. The first sound is owing to the contractions of the large
mass of muscle composing the ventricles.
3. The sound is directly traceable to eddies in the blood.
Fig. 201.
Fig. 201.— Microscopic appearance of fibers from the heart. The cross-strite, divisions
(branching), and junctures are visible (Landois).
Fig. 202.— Muscular fiber-cells from the heart. (1 x 425.) a, line of juncture between
two cells; b. c, branching cells.
But, looking at the whole question broadly, is it not unrea-
sonable to explain the sound resulting from such a complex act
236 COMPARATIVE PHYSIOLOGY.
as the contraction of the heart and what it implies in the light
of any single factor ? That such narrow and exclusive views
should have been propagated, even by eminent physiologists,
should admonish the student to receive with great caution ex-
planations of the working of complex organs, based on a single
experiment, observation, or argument of any kind.
The view we recommend the student to adopt in the light of
our present knowledge is, that the first sound is the result of
several causative factors, prominent among which are the sud-
den tension of the auriculo-ventricular valves, and the contrac-
tion of the cardiac muscle, not leaving out of the account the
possible and probable influence of the blood itself through
eddies or otherwise; nor would we ridicule the idea that in
some cases, at all events, the sound may be modified in quality
and intensity by the shock given to the chest wall during sys-
tole.
ENDO-CARDIAC PRESSURES.
Bearing in mind the relative extent of the pulmonary and
systemic portions of the circulation, we should suppose that the
resistance to be overcome in opening the aortic valves and lift-
ing the column of blood that keeps them pressed together,
would be much greater in the left ventricle than in the right ;
or, in other words, that the intra-ventricular pressure of the
left side of the heart would greatly exceed that of the right, and
this is confirmed by actual experiment.
That there should be a negative pressure in, say, the left
ventricle, follows naturally enough from the fact that not only
are the contents of the ventricle expelled with great sudden-
ness, but that its walls remain (see Figs. 200 and 204) pressed
together for a considerable portion of the time occupied by the
whole systole ; so that in relaxation it follows that there must
be an empty cavity to fill, or that there must be an aspiratory
effect toward the ventricle; hence also one factor in the closure
of the semilunar valves.
It thus appears that the heart is not only a force-pump but
also to some extent a suction-pump ; and, if so, the aspirating
effect must express itself on the great veins, lacking valves as
they do, at their entrance into the heart ; hence, with each
diastole the blood would be sucked on into the auricles, a
result that is intensified by the respiratory movements of the
thorax.
THE CIRCULATION OF THE BLOOD.
237
Fig. 203. — Diagram showing the relative height of the blood-pressure in different parts
of the vascular system (after Yeo). h, heart; a, arterioles ; v, small veins; A,
arteries ; C, capillaries ; V, large veins; H, V, representing the zero-line, i. e., at-
mospheric pressure ; the blood-pressure is indicated by the height of the curve.
The numbers on the left give the pressure approximately, in mm. of mercury.
Relative Time occupied by the Various Phases of the Cardiac
Cycle. — The old and valuable diagram reproduced below is
meant to convey through the eye the relations of the main
events in a complete
beat of the heart or
cardiac cycle. The
relative length of the
sounds; the long peri-
od occupied by the
pause ; the duration of
the ventricular sys-
tole, which it is to be
observed is in excess
of that of the first
sound, are among the
chief facts to be noted.
The tracings of
Chauveau and Marey,
obtained from the
heart Of the horse, Fig. 204.— Diagram representing the movements and
, . , , , sounds of the heart during a cardiac cycle (after
which has a very slow sharpey).
238 COMPARATIVE PHYSIOLOGY.
rhythm, show that of the whole period, the auricular systole
occupies £ or Tao of a second ; the ventricular systole, f or T\ of
a second ; and the diastole, f or T% of a second.
With the more rapid beat in man (70 to 80 per minute), the
duration of the cardiac cycle may be estimated at about ■& of
a second, and the probable proportions for each event are about
these : The auricular systole, TV of a second ; the ventricular
systole, T3o of a second ; and the pause, T*o of a second.
It will be noted that the pause of the heart is equal in dura-
tion to the other events put together ; and even assuming that
there is some expenditure of energy in the return (relaxation)
of the heart to its passive form, there still remains a consider-
able interval for rest, so that this organ, the very type of cease-
less activity, has its periods of complete repose.
THE WORK OF THE HEART.
Since the pressure against which the heart works must, as
we shall see, vary from moment to moment, and sometimes
very considerably, the work of the heart must also vary within
wide limits, even making allowance for large adaptability to
the burden to be lifted ; for it will be borne in mind that the de-
gree to which the heart empties its chambers is also variable.
If one knew the quantity of blood ejected by the left ven.
tricle, and the rate of the beat, the calculation of the work done
would be an easy matter, since the former multiplied by the
latter would represent, as in the case of a skeletal muscle, the
work of the muscles of the left ventricle ; from which the work
of the other chambers might be approximately calculated.
The work of the auricles must be slight. The right ven-
tricle, it is estimated, does from one fourth to one third the
work of the left.
When we calculate the work done by the heart for certain in-
tervals, as the day, the week, month, year, and especially for a
moderate lifetime, and compare this with that of any machine it
is within the highest modern skill to construct, the great superi-
ority of the vital pump in endurance and working capacity will
be very apparent ; not to take into the account at all its wonder-
ful adaptations to the countless vicissitudes of life, without which
it would be absolutely useless, even destructive to the organism.
Some of these variations in the working of the heart we may
now to advantage consider.
THE CIRCULATION OF THE BLOOD. 239
VARIATIONS IN THE CARDIAC PULSATION.
These may be ascertained either by the investigation of the
arteries or of the heart, for every considerable alteration in the
working of the heart expresses itself also through the arterial
system. In speaking of the pulse, the reference is principally
to the arteries, but in each case we may equally well think of
the heai-t primarily as acting upon the arteries.
1. The frequency of the heart-beat varies, as might be sup-
posed, with a great multitude of conditions, the principal of
which are : age, being most frequent at birth, gradually slow-
ing to old age, while in feeble old age the heart-beat may, like
many other of the functions of the body, approximate the con-
dition at birth, being very frequent, small, feeble, and easily
disturbed in its rhythm ; sex, the cardiac beat being more fre-
quent in females ; posture, most rapid in the standing position,
slower when sitting, and slowest in the recumbent attitude;
season, more frequent in summer ; period of the day, more
frequent in the afternoon and evening. Elevation of tem-
perature, the inspiratory act, emotions, and mental activity,
eating, muscular exercise, etc., render the heart-beats more
frequent.
2. The length of the systole, though variable, is more con-
stant than that of the diastole.
3. Tlie force of the pulsation varies very greatly and exer-
cises an important influence on the blood-pressure and the
velocity of the blood-stream. As a rule, when the heart beats
rapidly, especially for any considerable length of time, the
force of the individual pulsations is diminished.
4. The heart-beat may vary much and in ways it is quite
possible to estimate, either directly by the hand placed over the
organ on the chest, by the modifications of the cardiac sounds,
or by the use of instruments. It is wonderful how much in-
formation may be conveyed, without the employment of any
instruments, through palpation and auscultation, to one who
has long investigated tbe heart and the arteries with an intelli-
gent, inquiring mind; and we strongly recommend the student
to commence personal observations early and to maintain them
persistently.
Practitioners recognize the pulse (and heart) as " slow " as dis-
tinguished from " infrequent," " slapping," " heaving," " thrill-
ing," u bounding," etc.
240
COMPARATIVE PHYSIOLOGY.
Now, if with these terms there arise in the mind correspond-
ing mental pictures of the action of the heart under the cir-
cumstances, well ; if not, there is a very undesirable blank.
How the student may be helped to a knowledge of the actual
behavior of the heart under a variety of conditions we shall
endeavor to explain later.
Apart from all the above peculiarities, the heart may cease
its action at regular intervals, or at intervals which seem to
possess no definite relations to each other — that is, the heart
may be irregular in its action, which may be made evident
either to the hand or the ear.
There are certain deviations from the quicker rhythm whicb
occur with such regularity and are so dependent on events that
take place in other parts of the body that they may be con-
sidered normal.
Comparative. — We strongly recommend the student to verify
all the statements made in these sections by direct observation
for himself. Such is invaluable to the practitioner. The fol-
lowing table gives the mean number of cardiac pulsations per
minute (after Gamgee) :
SPECIES.
Adult.
Horse 36- 40
Ass and mule 46- 50
Ox 45-50
Sheep and goat 70-80
Pig. 70- 80
Dog 90-100
Cat 120-140
Youth.
60- 72
65- 75
60- 70
85- 95
100-110
110-120
120-140
Old age.
32- 38
55- 60
40- 45
55- 60
55- 60
60- 70
100-120
The variations with age, for the horse and the ox, are as fol-
lows, according to Kreutzer :
Horse. Ox.
At birth 100-120
When 14 days old 80-96
When 3 months old 68- 76
When 6 months old 64- 72
When 1 year old 48-56
When 2 years old 40-48
When 3 years old 38-48
When 4 years old 38-50
When aged 32- 40
At birth 92-132
When 4-5 days old 100-120
When 14 days old 68
When 4-6 weeks old 64
When 6-12 months old . . 56- 68
For the young cow 46
For the four-year-old ox . 40
THE CIRCULATION OF THE BLOOD.
241
THE PULSE.
Naturally the intermittent action of the heart gives rise to
corresponding phenomena in the elastic tubes into which it
may he said to be continued, for it is very desirable to keep in
mind tbe complete continuity of the vascular system.
The following phenomena are easy of observation : When
a finger- tip is laid on any artery, an interrupted pressure is felt ;
if the vessel be laid bare (or observed in an old man), it may
be seen to be moved in its bed forward and upward ; the press-
ure is less the farther the artery from the heart ; if the vessel
be opened, blood flows from it continuously, but in spurts ; if
one finger be laid on the carotid and another on a distant ves-
sel, as one of the arteries of the foot, it may be observed (though
it is not easy from difficulty in attending to two events hap-
pening so very close together) that the beat in the nearer ves-
sel precedes by a slight interval that in the more distant.
Investigating the latter phenomenon with instruments, it is
found tbat an appreciable interval, depending on the distance
apart of the points observed, intervenes.
What is the explanation of these facts ?
The student may get at this by a few additional observa-
tions that can be easily made.
Fig. 205. — Marey's apparatus for showing the mode in which the pulse is propagated
in the arteries. B. a rubber pump, with valves to prevent regurgitation. The
working of the apparatus will be apparent from the inspection of the figure.
If water be sent through a long elastic tube (so coiled that
points near and remote may be felt at the same time) by a bulb
1(3
242 COMPARATIVE PHYSIOLOGY.
syringe, imitating the heart, and against a resistance made by
drawing out a glass tube to a fine point and inserting it into
the terminal end of the rubber tube, an intermittent pi'essure
like that occurring in the artery may be observed ; and further
that it does not occur at precisely the same moment at the two
points tested.
Information more exact, though possibly open to error, may
be obtained by the use of more elaborate apparatus and the
graphic method.
By measurement it has been ascertained that in man the
pulse-wave travels at the rate of from five to ten metres per sec-
ond, being of course very variable in velocity. It would seem
that the more rigid the arteries the more rapid the rate, for in
children with their more elastic arteries the speed is slower ;
and the same principle is supposed to explain the higher veloci-
ty noticed in the arteries of the lower extremities. But with
such a speed as even five metres a second it is evident that with
a systole of moderate duration (say '3 second) the most distant
arteriole will have been reached by the pulse-wave before that
systole is completed.
It is known that the blood-current at its swiftest has no
such speed as this, never perhaps exceeding in man half a metre
per second, so that the pulse and the blood-current must be two
totally distinct things.
When the left ventricle throws its blood into vessels already
full to distention, there must be considerable concussion in con-
sequence of the rapid and forcible nature of the cardiac systole,
and this gives rise to a wave in the blood which, as it passes
along its surface, causes each part of every artery in succession
to respond by an elevation above the general level, and it is this
which the finger feels when laid upon an artery.
That there is considerable distention of the arterial system
with each pulse may be realized in various ways, as by watch-
ing and feeling an artery laid bare in its course, or in very thin
or very old people, and by noticing the jerking of one leg
crossed over the other, by which method in fact the pulse-rate
may be ascertained. And that not only the whole body but
the entire room in which a person sits is thrown into vibra-
tion by the heart's beat, may be learned by the use of a tele-
scope to observe objects in the room, which may thus be seen to
be in motion.
Features of an Arterial Pulse-Tracing.— In order to judge of
THE CIRCULATION OF THE BLOOD.
243
the nature of arterial tracings, it is important that the circum-
stances under which they are obtained should be known.
;. »uo. — luarey s improveu spnygmograpn arranged lor iaK
spring; B, first lever; C, writing lever; 6", its free writing end; D\ screw for
bringing B in contact with C\ G, slide with smoked paper; H, clock-work; L,
screw for increasing the pressure; M, dial indicating the amount of pressure,
K, A", straps for fixing the instrument to the arm, and the latter to the double-
inclined plane or support (Byrom Bramwell).
The movements of the vessel wall in most mammals suitable
for experiment aud in man is so slight that it becomes necessary
to exaggerate them in the tracing, hence long levers are used to
accomplish this.
The sphygmograph is the usual form of instrument em-
ployed for the purpose. It consists, essentially, of a clock-work
for moving a smoked surface
(mica plate commonly) on
which the movements of a
lever-tip, answering to those
of a button placed on the
artery, are recorded.
We shall do well to in-
quire whether there are any
features in common in trac-
ings obtained in various
ways, and which have there-
fore in all probability a real foundation in nature.
An inspection of a large number of pulse- tracings, taken un-
der diverse conditions, seems to show that in all of them there
occurs, more or less marked, the following : 1. An upward
Fig. 207.— Diagrammatic schema showing the
essential part of the instrument when in
working order. The knife-edge B" of
the short lever is in contact with the
writing-lever C. Every movement of the
steel spring at A", communicated by the
arteries, will be imparted to the writing-
lever (Byrom Bramwell).
244
COMPARATIVE PHYSIOLOGY.
curve. 2. A downward curve, rendered irregular by the occur-
rence of peaks or crests and notches. The first of these are
Fig, 208.— Pulse tracing from carotid artery of healthy man (after Moens). x, com-
mencement of expansion of artery; A, summit of first rise; C, dicrotic secondary
wave; B, predicrotic secondary wave; p, notch preceding this; D, succeeding sec-
ondary wave. Curve above is that made by a tuning-fork with ten double vibra-
tions in a second.
termed the predicrotic notch and crest, and the succeeding ones
the dicrotic notch and crest. The latter seem to be the more
constant.
Venous Pulse. — Apart from the variations in the caliber of
the great veins near the heart, constituting a sort of pulse,
though due to variations in intra-cardiac pressure, a venous
pulse proper is rare as a normal feature. One of the best-known
examples of such occurs in the salivary gland. When, during
secretion, the arterioles are greatly dilated, a pulse may be wit-
nessed in the veins into which the capillaries open out, owing
to diminution in the resistance which usually is sufficiently
great to obliterate the pulse-wave.
Pathological. — In severe cases of heart-disease, owing to
cardiac dilatation or other conditions, giving rise to incompe-
tency of the tricuspid valves, there may be with each ventricu-
lar systole a back-flow, visible in the veins of the neck.
A venous pulse is a phenomenon, it will be evident, that
always demands special investigation. It means that the usual
bounds of nature are for some good reason being overstepped.
Comparative. — Before entering on the consideration of phe-
nomena that all are agreed are purely vital, we call attention to
the circulation in forms lower than the mammal, in order to
give breadth to the student's views and prepare him for the
special investigations, which must be referred to in subsequent
chapters; and which, owing to the previous narrow limits (re-
searches upon the frog and a few well-known mammals) having
at last been overleaped, have opened up entirely new aspects of
THE CIRCULATION OP THE BLOOD. 215
cardiac physiology — one might almost say revolutionized the
subject.
Owing to the limitations of our space, the references to lower
forms must be brief.
We recommend the student, however, to push the subject
further, and especially to carry out some of the experiments to
which attention will be directed very shortly.
In the lowest organisms {Infusorians) represented by Amoe-
ba, Vorticella, etc., there are, of course, no circulatory organs,
unless the pulsating vacuoles of some forms mark the crude
beginnings of a heart. It will be borne in mind, bowever, that
there is a constant streaming of the protoplasm itself within
the organism.
The heart is first represented, as in worms, by a pulsatile
tube, which may, as in the earth-worm, extend throughout the
greater part of the length of the animal, and has usually dorsal
and ventral and transverse connections
The dilatations of the transverse portions in one division
(metamere) of the animal seem to foreshadow the appearance of
auricles.
The pulsation of the dorsal vessel in a large earth-worm is
easy of observation.
In amphioxus, which is often instanced as the lowest verte-
brate, the blood-vessels, including the portal vein, are pulsatile,
while there is no distinct and separate heart.
Although the respiratory system will be treated from the
comparative point of view, the student will do well to note now
Ab Ao 4'
Ba V ^7F
Fig. 209.— Diagram of the circulation of a Teleostean fish (Clans). V. ventricle; Ba,
bulbus arteriosus, with the arterial arches which carry the blood to the gills; Ab,
arterial arches: Jo, aorta descendens, into which the epibranchiaJ arteries passing
out from the gills unite; K, kidneys; /, intestine; Pc. portal circulation.
(in the figures) the close relation between the organs for dis-
tributing and aerating the blood.
Passing on to the vertebrates, in the lowest group, the fishes,
246
COMPARATIVE PHYSIOLOGY.
the heart consists of two chambers, an auricle and a ventricle,
the latter being supplemented by an extension (bulbus arterio-
sus) pulsatile in certain species ; and an examination of the
Fig. 210.
Pig. 211.
Fig. 210. — The arterial trunks and their main branches in the frog {Rana esculenta).
1 x 1|. (Howes.) I, lingual vessel; c. c, common carotid artery; p.cu, pulmo-
cutaneous artery; c. gl, carotid gland; aw', right auricle; aw", left auricle; v, ven-
tricle; tr.a, truncus arteriosus; pul', pulmonary; Ig, left lung; ao, left aortic
arch; br, brachial; cu) cutaneous; d. ao, dorsal aorta; cm, coeliaco-mesenteric;
cm', coeliac; hn, hepatic vessels; 17, gastric; j>c', pancreas; m, mesenteric; sp,
splenic; du', duodenal; h, hajmorrhoidal; W, ileal; h,y, hypogastric; c. II, com-
mon iliac; re, renal; k, kidney; Is, spermatic.
Fig. 211.— Venous trunks and their main branches in the frog (Rana esculenta). 1 x 1J.
(Howes.) I, lingual vein; e.j, external jugular; in, innominate; i.j, internal jugu-
lar; s. sc, subscapular; pr. c, vena cava superior; s. v, sinus venosus; hp, hepatic;
I11', right lobe of liver; Iv", left lobe of liver; pt. c, vena cava inferior; ov, ovarian;
d. I, dorso-lumbar; od, oviducal; r.p, renal-portal; fm, femoral; sc, sciatic; a,
femoro-sciatic anastomosis; pv' , right pelvic; vs, vesical; ant. ab, anterior ab-
dominal; a', abdominal-portal anastomosis; W, ileal; sp, splenic; du', duodenal;
I. int, lieno-intestinal; g, gastric; p, portal; Ig", left lung; pul, pulmonary; m. cu,
musculo-cutancous; br, brachial.
course of the circulation will show that the heart is throughout
venous, the blood being oxidized in the gills after leaving the
former.
Among the amphibians, represented by the frog, there are
two auricles separated by an almost complete septum, and one
THE CIRCULATION OF THE BLOOD.
247
ventricle characterized by a spongy arrangement of the muscle-
fibers of its walls.
In the reptiles the division between the auricles is complete,
and there is one ventricle which shows imperfect subdivisions.
Fig. 212.
Fig 213.
Fig. 212.— The frog's heart, seen from the front, the aortic arches of the left side hav-
ing been removed. (1 x 4.) ca, carotid; c. ffl, carotid gland; ao, aorta; an', right
auricle; au", left auricle; pr. c, vena cava superior; pt. c, vena cava inferior;
p. cu, pulmo-cutaneous trunk; tr, truncus arteriosus: v, ventricle (Howes).
Fig. 213.— The same, seen from behind, the sinus venosus having been opened up 10
show the sinu-auricular valves. (1 x 4.) p.v, pulmonary vein ; s. v, sinus veno-
bus; va", sinu-auricular valve. Other lettering as in Fig. 212 (Howes).
In the crocodile, however, the heart consists of four per-
fectly divided chambers. Of the two aortic arches, one arises
together with the pulmonary artery from the right ventricle,
and, as it crosses over, the left communicates with it by a small
opening, so that, although the arterial and the venous blood
are completely separated in the heart, they intermingle outside
of this organ.
In birds the circulatory system is substantially the same as
in mammals ; but in all vertebrate forms below birds the blood
distributed to the tissues is imperfectly oxidized or is partially
venous.
As a result of the entire vascular arrangements in the frog,
etc., the least oxidized blood passes to the lungs, and the most
aerated to the head and anterior parts of the animal.
Whatever ground for differences of opinion there may be
as to the extent to which the phenomena we have as yet been
describing are mechanical in their nature, all are agreed that
24S COMPARATIVE PHYSIOLOGY.
such explanations are insufficient when applied to the facts
with which we have yet to deal. They, at all events, can be
regarded only as the result of vitality.
When one reflects upon the vicissitudes through which an
animal must pass daily and hourly, necessitating either that
they be met by modified action of the organs of the body or
that the destruction of the organism ensue, it becomes clear that
the varying nutritive needs of each part must be answered by
changes in the circulatory system. These changes may affect
any part of the entire arrangement, and it rarely happens, as
will appear, that one part is modified without a corresponding
one, very frequently of a different kind, taking place in some
other. What these various correlated modifications are, and
how they are brought about, we shall now attempt to describe,
and it will greatly assist in the comprehension of the whole if
the student will endeavor to keep a clear mental picture of the
parts before his mind throughout, using the figures and verbal
descriptions only to assist in the construction of such a mental
image. We shall begin with the vital pump — the heart.
THE BEAT OF THE HEART AND ITS MODIFICATIONS.
As has been already noted, the cardiac muscle has features
peculiar to itself, and occupies histologically an intermediate
place between the plain and the striped muscle-cells, and that
the contraction of the heart is also intermediate in character,
and is best seen in those fox*ms of the organ which are somewhat
tubular and beat slowly. But the contraction, though peristal-
tic, is more rapid than is usually the case in organs with the
smooth form of muscle-fiber.
The heart behaves under a stimulus in a peculiar manner,
the effect of a single induction shock depends on the phase of
contraction hi which the heart happens to be at the moment of
its application. Thus at the commencement of a systole there
is no visible effect, while beats of unusual character result at
other times. But tetanus can not be induced by any form or
method of stimulation. The latent period of cardiac muscle is
long.
In a heart at rest a single stimulus (as the prick of a needle)
usually calls forth but one contraction.
THE CIRCULATION OF THE BLOOD. 249
THE NERVOUS SYSTEM IN RELATION TO THE HEART.
The attempts to determine just why the heart heats at all,
and especially the share taken by the nervous system, if any
direct one, are beset with great difficulty ; though, as we shall
attempt to show later, this subject also has been cramped within
too narrow limits, and hence regarded in a false light.
Till comparatively recently the frog's heart alone received
much attention, if we except those of certain well-known mam-
mals. In the heart of the frog there are ganglion-cells in vari-
ous parts, especially numerous in the sinus venosus (or expan-
sion of the great veins where they meet the auricles) ; also in the
auricles, more especially in the septum (ganglia of Remak), while
they are absent from the greater part of the ventricle, though
found in the auriculo-ventricular groove (ganglia of Bidder).
Recently it has been found that ganglion-cells occur in the
ventricles of warm-blooded animals. In the hearts of the dog,
calf, sheep, and pig, which are those lately subjected to investi-
gation, it is found that the nerve-cells do not occur near the
apex of the ventricles, but mainly in the middle and basal por-
tions, being most abundant in the anterior and posterior inter-
ventricular furrows and in the left ventricle. But there are
differences for each group of animals ; thus, these ganglion-
cells are most abundant, so far as the mammals as yet inves-
tigated ai'e concerned, in the ventricles of the pig, and least so
in those of the dog. In tbe cat they ai'e also scanty. Ganglion-
cells occur in the auricles, and are especially abundant near the
terminations of the great veins.
It has long been known that the heart of a frog removed
from the body will pulsate for hours, especially if fed with
serum, blood, or similar fluids ; and that it may be divided in
almost any conceivable way, even when teased up into minute
particles, and still continue to beat. The apex, however, when
separated does not beat. Yet even this quiescent apex may be
set pulsating if tied upon the end of a tube, through which it
may be fed under pressure.
We may here point out that the whole heart or a part of it
may be made to describe its action by the graphic method in
various ways, the principles underlying which are either that
the heart pulls upon a recording lever (lifts it) ; acts against the
fluid of a manometer ; or, inclosed in a vessel containing oil or
similar fluid, moves a piston in a cylinder.
250 COMPARATIVE PHYSIOLOGY.
It lias also long been known that a ligature drawn around
the sinus venosus (in the frog) at its junction with the auricles
stopped the heart for a certain period, and this experiment (of
Stannius) was thought to demonstrate that the heart was ar-
rested because the nervous impulses proceeding to the ganglion-
cells along the cardiac nerves or ganglia of this region were
cut off by the ligature ; in other words, the heart ceased to beat
because the outside machinery on which the action of the inner
depended was suddenly disconnected. Other explanations have
been offered of this fact.
Within the last few years great light has been thrown upon
the whole subject of cardiac physiology in consequence of in-
vestigators having studied the hearts of various cold-blooded
animals and of several invertebrates. The hearts of the Che-
lonians (tortoises, turtles) have received special attention, and
their investigation has been fruitful of results, to the general
outcome of which, as well as those accruing from recent com-
parative studies as a whole, we can alone refer. Since in other
parts of the work the limits of space will not always allow us
to give the evidence on which conclusions rest, attention is
especially called to what here follows, as an example of the
methods of physiological research, and the nature of the reason-
ing employed.
Very briefly the following are some of the main facts :
1. In all cold-blooded animals the order in which the sub-
divisions of the heart ceases to pulsate when kept under the
same conditions is invariable, viz., ventricle, auricles, sinus.
2. The sinus and auricles, when separated by section, liga-
ture, or otherwise, either together or singly, continue to beat,
whether amply provided with or surrounded by blood.
3. The ventricle thus separated displays less tendency to
beat independent of some stimulus (as feeding under pressure),
though a very weak one usually suffices — i. e., its tendency to
spontaneous rhythm is less marked than is the case with the
other parts of the heart. These remarks apply to the hearts
of Chelonians — fishes, snakes, and some other cold-blooded
animals.
4. In certain fishes (skate, ray, shark) the beat may be re-
versed by stimulation, as a prick of the ventricle. This is
accomplished with more difficulty in other cold-blooded ani-
mals, and still more so in the mammal.
5. In certain invertebrates, notably the Poulpe (Octopus), a
THE CIRCULATION OF THE BLOOD. 251
careful search has revealed no nerve-cells, yet their hearts con-
tinue to beat when their nerves are severed, on section of parts
of the organ, etc.
6. A strip of the muscle from the ventricle of the tortoise,
when placed in a moist chamber and a current of electricity
passed through it for some hours, will commence to pulsate and
continue to do so after the current has been withdrawn ; and
this holds when the strip is wholly free from nerve-cells.
From the above facts certain inferences have been drawn :
1. It has been concluded that the sinus is the originator and
director of the movements of the rest of the heart. 2. That this
is owing to the ganglia in its walls. While all recognize the
importance of the sinus, some physiologists hold to the gangli-
onic influence as essential to the heart-beat still ; while others,
influenced by the facts mentioned above, are disposed to regard
tbem as of very doubtful importance — at all events, as origina-
tors of the movements of the heart.
The tendency now seems to be to attach undue importance
to the spontaneous contractility of the heart-muscle ; for it by
no means follows logically that, because a muscle treated by
electricity, when cut off from the usual nerve influence that we
believe is being constantly exerted on the heart like other or-
gans, will contract and continue to do so in the absence of the
stimulus, it does so normally.; or, because some hearts beat in
the absence of nerve-cells, that therefore nerve-cells are of no
account in any case. Such views, when pressed to the extreme,
lead to as narrow conceptions as those they are intended to re-
place.
Taking into account the facts mentioned and others we have
not space to enumerate, we submit the following as a safe view
to entertain of the beat of the heart in the light of our present
knowledge :
Recent investigations show clearly that there are great dif-
ferences in the hearts of animals of diverse groups, so that it
is not possible to speak of "the heart" as though our remarks
applied equally to this organ in all groups of animals.
It must be admitted that our understanding of the hearts of
the cold-blooded animals is greater than of the mammalian
heart ; while, so far as exact or experimental knowledge is con-
cerned, the human heart is the least understood of all, though
there is evidence of a pathological and clinical kind and subject-
ive experience on which to base conclusions possessing a certain
252 COMPARATIVE PHYSIOLOGY.
value ; but it is clear to those who have devoted attention to
comparative physiology that the more this subject is extended
the better prepared we shall be for taking a broad and sound
view of the physiology of the human heart and man's other
organs.
Whatever may be said of the invertebrates, among which
greater simplicity of mechanism doubtless prevails, there can
be no doubt that the execution of a cardiac cycle of the heart
in all vertebrates, and especially in the higher, is a very com-
plex process from the number of the factors involved, their in-
teraction, and their normal variation with circumstances ; and
we must therefore be suspicious of any theory of excessive sim-
plicity in this as well as other parts of physiology.
We submit, then, the following as a safe provisional view of
the causation of the heart-beat :
1. The factors entering into the causation of the heart-beat
of all vertebrates as yet examined are : (a) A tendency to spon-
taneous contraction of the muscle-cells composing the organ:
(6) intra-carcliac blood-pressure ; (c) condition of nutrition as
determined directly by the nervous supply of the organ and in-
directly by the blood.
2. The tendency to spontaneous contraction of muscle-cells
is most marked in the oldest parts of the heart (e.g., sinus),
ancestrally (phylogenetically) considered.
3. Inti*a-carcliac pressure exercises an influence in determin-
ing the origin of pulsation in probably all hearts, though like
other factors its influence varies with the animal group. In
the mollusk (and allied forms) and in the fish it seems to be the
controlling factor.
4. We must recognize the power one cell has to excite, when
in action, neighboring heart-cells \o contraction. The ability
that one protoplasmic cell-mass has to initiate in others, under
certain circumstances, like conditions with its own, is worthy
of more serious consideration in health and disease than it has
yet received.
5. The influence of the cardiac nerves becomes more pro-
nounced as we ascend the animal scale. Their share in the
heart's beat will be considered later.
6. Apparently in all hearts there is a functional connection
leading to a regular sequence of beat in the different parts, in
which the sinus or its representatives (the terminations of great
veins in the heart) always takes the initiative. One part having
THE CIRCULATION OF THE BLOOD.
253
contracted, the others must necessarily follow ; hence the rapid
onset of the ventricular after the auricular contraction in the
mammal, and the long wave of contraction that seems to pass
evenly over the whole organ in cold-blooded animals.
The basis of all these factors is to be sought finally in the
natural contractility of protoplasm. A heart in its most de-
veloped form still retains, so to speak, the inherited but modified
Amoeba in its every cell.
Whether the intrinsic nerve-cells of the heart take any share
directly in the cardiac beat must be considered as yet undeter-
mined. Possibly they do modify motor impulses from nerves,
while again it may be that they have an influence over nutri-
tive processes only. The subject requires further study, both
anatomical and physiological.
INFLUENCE OF THE VAGUS NERVE UPON THE HEART.
The principal facts in this connection may be stated as fol-
lows, and apply to all the animals thus far examined :
1. In all cases the action of the heart is modified by stimu-
lation of the medulla oblongata or the vagus nerve.
2. The modification may consist in prompt arrest of the
heart, in slowing, in enfeeblement of the beat, or a combination
of the two latter effects.
3. After the application of the stimulation there is a latent
Fig. 214.— Inhibition of frog's heart by stimulation of the vagus nerve. To be read
from right to left. The contractions of the ventricle are registered by a simple
lever resting on it. The interrupted current was thrown in at a. Note that one
beat occurred before arrest (latent period), and that when standstill of the heart
did take place it lasted for a considerable period (.Foster).
period before the effect is manifest, and the latter may outlast
the stimulation by a considerable period.
254 COMPARATIVE PHYSIOLOGY.
4. In most animals the sinus venosus and auricles are af-
fected before the ventricles, and the vagus may influence these
parts when it is powerless over the ventricle.
5. After vagus inhibition, the action of the heart is (almost
unexceptionally) different, the precise result being variable, but
generally the beat is both accelerated and increased in force.
We may say that the werking capacity of the heart is tem-
porarily increased.
6. The improvement in the efficiency of the heart is in pro-
portion to its previous working power, and in cases when the
Stimulation Vagus.
Fig. 915.— Effects of vagus stimulation, illustrated by a form of sphygmograpliic curve
derived from the carotid of a rabbit (Foster).
action is feeble and irregular (abnormal) it might be said to be
in proportion to its needs. This is a very important law that
deserves to receive a general recognition.
7. Section of both vagi nerves results in histological altera-
tions in the heart's structure, chiefly fatty degeneration, which
must, of course, impair its working capacity and expose it
to rupture or other accidents under the frequently recurring
strains of life.
8. In the cold-blooded animals the heart may be kept at a
standstill by vagus stimulation till it dies, a period of hours
(one case of six hours reported for the sea-turtle).
9. Certain drugs (as atropine), applied directly to the heart,
or injected into the blood, prevent the usual action of the vagus.
10. During vagus arrest the heart substance undergoes a
change, resulting in an unusual dilatation of the organ. This
may be witnessed whether the heart contains blood or not.
THE CIRCULATION OP THE BLOOD. 255
11. The heart may he arrested by direct stimulation, espe-
cially of the sinus, and at the points at which the electrodes are
applied there is apparently a temporary paralysis. The same
alteration in the beat may he noticed as when the main trunk
of the vagus is stimulated.
12. The heart may he inhibited through stimulation of vari-
ous parts of the body, both of the surface and internal organs
(reflex inhibition).
13. One vagus being divided, stimulation of its upper end
may cause arrest of the heart.
14. Stimulation of a small part of the medulla oblongata
will produce the same result, provided one or both vagi be
intact.
15. Section of both vagi in some animals (the dog notably)
increases the rate of the cardiac beat. The result of section of
one pneumogastric nerve is variable. The heart's rhythm is
usually to some extent quickened.
16. During vagus inhibition from any cause in mammals
and many other animals, the heart responds to a single stimu-
lus, as the prick of a needle, by at least one beat. An observer
studying for himself the behavior of the heart in several groups
of animals with an open mind, for the purpose of observing all
he can rather than proving or disproving some one point, be-
comes strongly impressed with the variety in unity that runs
through cardiac physiology, including the influence of nerve-
cells (centers) through nerves ; for it will not be forgotten that
normally nerves originate nothing, being conductors only, so
that when the vagus is stimulated by us we are at the most but
imitating in a rough way the work of central nerve-cells. We
can only mention a few points to illustrate this.
In the frog a succession of light taps, or a single sharp one
(" Klopf versuch " of Goltz), will usually arrest the heart reflexly ;
though sometimes it is very difficult to accomplish. But in the
fish the ease with which the heart may be reflexly inhibited by
gentle stimulation of almost any portion of the animal is won-
derful. Again, in some animals the vagus arrests the heart for
only a brief period, when it breaks away into its usual (but in-
creased) action.
In the fish, menobranchus, and probably other animals, the
irritability of some subdivision of the heart is lost during the
vagus inhibition — i. e., it does not x-espond to a mechanical
stimulus.
256
COMPARATIVE PHYSIOLOGY.
There is usually a certain order in which the heart recom-
mences after inhibition (viz. , sinus, auricles, ventricles) ; but
there are variations in this, also, for different animals. It is
also a fact that in most of the cold-blooded animals the right
S. Vagus,
Heart.
Brain above Medulla.
Cardie-inhibitory Cen-
ter in Medulla Ob-
Afferent Nerve.
Outlying Area with its
Nerves.
Fio. 310. — Diagram of the inhibitory mechanism of the heart. The arrows indicate
in all cases the path the nervous impulses take. I. Path of afferent impulses
from I he heart itself. II. Path from parts of the brain above (or anterior to) the
vaso-motor center. A similar one might, of course, be mapped out along the
spinal cord. III. Path from some peripheral region. The downward arrows in-
dicate the course of efferent impulses, which probably usually pass by both vagi.
vagus is more efficient than the left, owing, we think, not to
the nerves themselves so much as to their manner of distribu-
tion in the heart—the greater portion of the driving part of the
THE CIRCULATION OP THE BLOOD. 257'
organ,so to speak, being supplied by tbe right nerve ; for, when
even a small part of the heart is arrested, it may be overcome by
the action of a larger portion of the same, or a more dominant
region (the sinus mostly).
Conclusions. — The inferences from tbe facts stated in the
above paragraphs are these : 1. There is in the medulla a col-
lection of cells (center) which can generate impulses that reach
the heart by the vagi nerves and influence its muscular tissue,
though whether directly or through the intermediation of
nerve-cells in its substance is uncertain. It may possibly be in
both ways. 2. This center (cardio-inhibitory) may be influ-
enced reflexly by influences ascending by a variety of nerves
from the periphery, including paths in the brain itself, as
shown by the influence of emotions or the behavior of the
heart. 3. The cardio-inhibitory center is the agent, in part,
through which the rhythm of the heart is adapted to the needs
of the body. 4. The arrest, on direct stimulation of the heart,
is owing to the effect produced on the terminal fibers of the
vagi, as shown by the dilatation, etc., corresponding to what
takes place when the trunk of the nerve or the center is stimu-
lated. 5. The quickening of the heart, following section of the
vagi, seems to show that in some animals the inhibitory center
exercises a constant regulative influence over the rhythm of
the heart. 6. The irritability and dilatability of the cardiac
tissue may be greatly modified dui'ing vagus inhibition. Some-
times this is evident before the rhythm itself is appreciably
altered. 7. The heart-muscle has a latent period, like other
kinds of muscle ; and cardiac effects, when initiated, last a vari-
able period.
There are many other obvious conclusions, which the stu-
dent will draw for himself.
But a question arises in regard to the significance of the
cardiac arrest under these circumstances, and the altered action
that follows. The fact that, when the heart is severed from the
central nervous system by section of its nerves, profound
changes in the minute structure of its cells ensue, points un-
mistakably to some nutritive influence that must have operated
through the vagi nerves. That stimulation of the vagus re-
stores regularity of rhythm and strengthens the beat of the
failing heart, is also very suggestive. That many disorders of
the heart are coincident with periods of mental anguish or
worry, and that in certain cases of severe mental application
17
258 COMPARATIVE PHYSIOLOGY.
the heart's rhythm lias become very slow, also point to influ-
ences of a central origin as greatly affecting the life-processes
of this organ.
It has been shown that the vagus nerve in some cold-blooded
animals, as is probable also in the higher vertebrates, consists
of two sets of fibers — those which are inhibitory proper and
those which are not, but belong to the sympathetic system.
Sepai'ate stimulation of the former favors nutritive processes,
is preservative ; of the latter, destructive. This has been ex-
pressed by saying that the former favors constrictive (anabolic)
metabolism ; the latter destructive (katabolic) metabolism. It
is assumed that all the metabolism of the body may be repre-
sented as made up of katabolic following anabolic processes.
Whether such a view of metabolism expresses any more
than a sort of general tendency of the chemistry of the body
is doubtful. It is a very simple representation of what in all
probability is extremely complex ; and if it be implied that
throughout the body certain steps are always taken upward in
construction to be always afterward followed by certain down-
ward destructive changes, we must reject it as too rigid and
artificial a representation of natural processes.
We think, however, that, upon all the evidence, pathological
and clinical as well as physiological, the student may believe
that the vagus nerve, like the other nerves of the body, accord-
ing to our own theory, exercises a constant beneficial, guiding
— let us say determining — influence over the metabolism of the
organ it supplies ; and we here suggest that, if this view were
applied to the origin and course of cardiac disease, it would
result in a gain to the science and art of medicine.
THE ACCELERATOR (AUGMENTOR) NERVES OF THE
HEART.
It has been known for many years that in the dog, cat,
rabbit, and some other mammals, there are nerves proceeding
from certain of the ganglia of the sympathetic chain high up,
stimulation of which lead to an acceleration of the heart-beat.
Very recently these nerves have been traced in a number of
cold-blooded animals, and the whole subject placed on a broader
and sounder basis.
There are variations in the distribution of these nerves for
different groups of animals, but it will suffice if we indicate
THE CIRCULATION OF THE BLOOD.
259
their course in a general way, without special reference to the
variations for each animal group: 1. These nerves emerge from
the spinal cord (upper dorsal region), and proceed upward
Spinal Cord.
Accelerator Center in Me-
dulla.
Superior Cervical Ganglion.
Middle Cervical Ganglion.
Inferior Cervical Ganglion.
Basal Ganglion in Region
of First Rib.
Accelerator Nerves.
Heart.
Fig. 217.^Diagrani to illustrate the origin, course, etc., of accelerator impulses. It
will be understood that this is intended to indicate the general plan, and not pre-
cisely what takes place in any one animal. Thus, while the accelerator nerves
may arise in this way, it is not meant to be implied that the heart is actually sup-
plied by three nerves of such origin in any case. The arrows, as before, indicate
the path of the impulses.
before being distributed to the heart. 2. They may leave for
their cardiac destination either at (a) the first thoracic (or basal
cardiac ganglion, as it might be named in this case), (6) the in-
ferior cervical ganglion, (c) the annulus of Vieussens, or (d) the
middle cervical ganglion.
It follows that the heart may be made to do increased work
in three ways : First, the relaxation of a normal inhibitory
260
COMPARATIVE PHYSIOLOGY.
control through the vagus nerve by the cardio-inhibitory cen-
ter ; second, through the sympathetic (motor) fibers in the
vagus itself : and, finally, through fibers with similar action
in the sympathetic system, usually so called.
The share taken by these factors is certainly variable in dif-
ferent species of animals, and it is likely that this is true of the
same animals on different occasions. It is also conceivable,
and indeed probable, that they act together at times, the inhibi-
tory action being diminished and the augmentor influence in-
creased.
THE HEART IN RELATION TO BLOOD-PRESSURE.
It is plain that all the other conditions throughout the cir-
culatory system remaining the same, an increase in either the
force or the frequency of the heart-beat must raise the blood-
pressure. But, if the pressure were generally raised when the
heart beats rapidly, it would fare ill with the aged, the elasticity
of their arteries being usually greatly impaired. As a matter of
fact any marked rise of pressure that would thus occur is pre-
>w~-s
Fig. 218 —Tracing from a rabbit, showing the influence of cardiac inhibition on blood-
pressure. The fall in this case was very rapid, owing to sudden cessation of the
heart-beat. The relative emptiness of the vessels accounts for the peculiar char-
acter of the curve of rising blood-pressure (Foster).
vented as a rule, and in different ways, as will be seen ; but, so
far as the heart is concerned, its beat is usually the weaker the
more rapid it is, so that the cardiac rhythm and the blood-press-
ure are in inverse proportion to each other.
By what method is the heart's action tempered to the condi-
THE CIRCULATION OF THE BLOOD. 261
tions prevailing at the time in the other parts of the vascular
system ?
The matter is complex. The effect of vagus stimulation on
the blood-pressure is always very marked, as would be supposed.
As seen in the tracing, the beats, when the heart commences
its action again tell on the comparatively slack walls of the
arteries, distending them greatly, and this may be made evident
by the sphymograph as well as the manometer ; indeed, may be
evident to the finger, the pulse resembling in some features that
following excessive loss of blood,
If the heart has been merely slowed, or its pulsation weak-
ened, the effects will of course be less marked.
The Quantity of Blood.— The blood-pressure may also be
augmented, the cardiac frequency remaining the same, by the
quantity of blood ejected from the ventricles, which again
depends on the quantity entering them, a factor determined
by the condition of the vessels, and to this we shall presently
turn.
In consequence of changes in different parts of the system by
way of compensation, results follow in an animal which might
not have been anticipated.
Thus, bleeding, unless to a dangerous extreme, does not lower
the blood-pressure except temporarily. It is estimated that the
body can adapt itself to a loss of as much as 3 per cent of the
body-weight.
The adaptation is probably not through absorption chiefly,
but through constriction of the vessels by the vaso- motor
nerves.
Again, an injection of fluid into the blood does not cause an
appreciable rise of blood-pressure, so long as the nervous svs-
tem is intact ; but, if by section of the spinal cord the vaso-
motor influences are cut off, then a rise may take place to the
extent of 2 to 3 per cent of the body -weight, the extra quan-
tity of fluid seeming to be accommodated in the capillaries and
smaller veins. These facts are highly significant in illustrat-
ing the adaptive power of the circulatory system (protective in
its nature), and are of practical importance in the treatment of
disease.
We think the benefit that sometimes follows bleeding has
not as yet received an adequate explanation, but we shall not
attempt to tackle the problem now. Changes in the circulation
depend on variations in the size of the blood-vessels.
262 COMPARATIVE PHYSIOLOGY.
It is important in considering this subject to have clear no-
tions of the structure of the blood-vessels. It will be borne in
mind that, while muscular elements are perhaps not wholly
lacking in any of the arteries, they are most abundant in the
smallest, the arterioles, which by their variations in size are best
fitted to determine the quantity of blood reaching any organ.
It is well known that nerves derived chiefly from the sympa-
thetic system pass to blood-vessels, though their exact mode of
termination is obscure. As the result of the section and stimu-
lation of certain nerves the following inferences have been
drawn in regard to the nerves supplying blood-vessels.
1. There are vaso-motor nerves of two kinds — vaso-constrict-
ors and vaso-dilators — which may exist in nerve-trunks either
separately or mingled. Examples of the former are found in
the cervical sympathetic, splanchnic, etc., of the latter in the
chorda tympani, nerves of the muscles and nervi erigentes
(from the first, second, and third sacral nerves), while the sci-
atic seems to contain both.
2. Impulses are constantly passing from the medullary vaso-
motor center along the nerves to the blood-vessels, hence their
dilatation after section of the nerves. The nerves are traceable
to the spinal cord, and in some part of their course run, as a
rule, in the sympathetic system.
3. Impulses pass at intervals to the areas of distribution of
vaso-dilators along these nerves, the effect of which is to dilate
the vessels through their influence, as in other cases, on the
muscular coat.
It is inferred that there are vaso-motor centers in the
spinal cord which are usually subordinated to the main center
in the medulla, but which in the absence of the control of the
chief center in the medulla assume an independent regulating
influence.
There is a nerve with variable origin, course, etc., in differ-
ent mammals, but in the rabbit given off from either the vagus,
the superior laryngeal, or by a branch from each, which, run-
ning near the sympathetic nerve and the carotid artery, reaches
the heart, to which it is distributed. This is known as the de-
pressor nerve.
From stimulation of the central end of this nerve results
follow which warrant the conclusion that impulses can by it
reach the vaso-motor center in the medulla, and interfere with
(inhibit) the outflow of efferent, constrictive, or tonic impulses,
THE CIRCULATION OP THE BLOOD.
263
which start from the vasomotor center, descend the cord, and
find their way to the organs of a definite region, in consequence
of which the muscular coats of the arterioles relax, more blood
flows to this area which is very large, and the general blood-
pressure is lowered.
Again, if the central end of one of the main nerves — e. g.,
sciatic — be stimulated, a marked change in the blood-pressure
iso-motor Center
in Medulla.
Spinal Cord
Efferent Vaso-mo
tor Nerve.
Outlying Vascularl
Area. ~v*-^!ti|ST
Afferent Nerve
from Periphery.
Fig. 219.— Diagram of nervous vasomotor mechanism. I. Course of afferent impulses
from the heart itself along the depressor nerve. II. Course from some other part
of the brain. III. Course from some peripheral region along a nerve joining the
spinal cord. The efferent impulses are represented as passing to a vascular area,
• reduced for the sake of simplicity to a single arteriole.
results, but whether in the direction of rise or fall seems to de-
pend upon the condition of the central nervous system, for, with
the animal under the influence of chloral, there is a fall; if
under urari, a rise,
264
COMPARATIVE PHYSIOLOGY.
It is not to be supposed that the change in any of these
cases is confined, to any one vascular area invariably, but that
it is this or that, according to the nerve stimulated, the condi-
J
fUUUUUUUUUVAJUUUUlAJlAAJLJLAJUUUULA^
Fig. 220.— Curve of blood-pressure resulting from stimulation of the central end of
the depressor nerve. To be read from right to left. T indicates the rate at which
the recording surface moved, the intervals denoting seconds. At V the current
was thrown into the nerve, and shut off at 0. The result appears after a period
of latency, and outlasts the stimulus (Poster)
tion of the centers, and a number of other circumstances.
Moreover, it is importaut to bear in mind that with a fall of
blood-pressure in one region there may be a corresponding rise
in another. With these considerations in mind, it will be ap-
parent that the changes in the vascular system during the
course of a single hour are of the most complex and variable
character.
The question of the distribution of vaso-motor nerves to
veins is one to which a definite answer can not be given.
THE CAPILLARIES.
The cells of which the capillaries are composed have a con-
tractility of their own, and hence the caliber of the capillaries
is not determined merely by the arterial pressure or any similar
mechanical effect.
Certain abnormal conditions, induced in these vessels by
the application of irritants, cause changes in the blood-flow,
which can not be explained apart from the vitality of the ves-
sels themselves.
Watched through the microscope under such circumstances,
THE CIRCULATION OP THE BLOOD. 265
the blood-corpuscles no longer pursue their usual course in the
mid-stream, but seem to be generally distributed and to hug the
walls, one result of which is a slowing of the stream, wholly
independent of events taking place in other vessels. It is thus
seen that in this condition (stasis) the capillaries have an in-
dependent influence essentially vital. We say independent, for
it is still an open question whether nerves are distributed to
capillaries or not. That inflammation, in which also the walls
undergo such serious changes that white and even red blood-
cells may pass through them {diapedesis), is not uninfluenced
by the nervous system, possibly induced through it in certain
cases, if not all, seems more than probable.
But when we consider the lymphatic system new light will,
it is hoped, be thrown upon the subject of the nature and the
influences which modify the capillaries. One thing will be
clear from what has been said, that even normally the capil-
laries must exert an influence of the nature of a resistance,
owing to their peculiar vital properties ; and, as we have already
intimated such considerations should not be excluded from any
conclusions we may draw in regard to tubes that are made up
of living cells, whether arteries, veins, or capillaries, though
manifestly the applicability to capillaries, with their less modi-
fied or more primitive structure, is stronger.
It has now become clear that the circulation may he modi-
fied either centrally or peripherally; that a change is never
purely local, but is correlated with other changes ; that the
whole is, in the higher animals, directly under the dominion of
the central nervous system ; and that it is through this part
chiefly that harmony in the vascular as in other systems and
with other systems is established. To have adequately grasped
this conception is worth more than a knowledge of countless
details.
SPECIAL CONSIDERATIONS.
Pathological —Changes may take place either in the sub-
stance of the cardiac muscle, in the valves, or in the blood-ves-
sels, of a nature unfavorable to the welfare of the body. Some
of these have been incidentally referred to already.
Hypertrophy, or an increase in the tissue of the heart, is
generally dependent on increased resistance, either within or
without the heart, in the region of the arterioles or capillaries.
Imperfections of the aortic valves may permit of regurgitation
266 COMPARATIVE PHYSIOLOGY.
of blood, entailing an extra effort if it is to be expelled in addi-
tion to the usual quantity, which again leads to hypertrophy ;
but this is often suceeded by dilatation of the chambers of the
heart one after the other, and a host of evils growing out of
this, largely dependent on imperfect venous circulation, and
increased venous pressure. And it may be here noticed that
arterial and venous pressures are, as a general rule, in inverse
proportion to each other.
If the quantity of blood in the ventricle, in consequence of
regurgitation, should prove to be greater than it can lift (eject),
the heart ceases to beat in diastole ; hence some of the sudden
deaths from disease of the aortic valves.
As a result of fatty, or other forms of degeneration, the
heart may suddenly rupture under strains.
Actual experiment on the arteries of animals recently dead,
including men, shows that the elasticity of the arteries of even
adult mammals is as perfect as that of the vessels of the child,
so that man ranks lower than other animals in this respect.
After a certain period of life the loss of arterial elasticity is
considerable and progressive. The arteries may undergo a de-
generation from fatty changes or deposit of lime ; such vessels
are, of course, liable to rupture ; hence one of the modes of
death among old animals is from paralysis traceable to rupture
of vessels in the brain.
These and other changes also cause the heart more work,
and may lead to hypertrophy. Even in young animals the
strain of a prolonged racing career may entail hypertrophy or
some other form of heart-disease.
We mention such facts as these to show the more clearly
how important is balance and the power of ready adaptation
in all parts of the circulation to the maintenance of a healthy
condition of body.
The heart is itself nourished through the coronary arteries ;
so that morbid alterations in these vessels cause, if not sudden
and painful death, at least nutritive changes in the heart-sub-
stance, which may lead to a dramatic end or to a slow impair-
ment of cardiac power, etc.
Personal Observation. — The circulation is one of those de-
partments of physiology in which the student may verify much
upon his own person. The cardiac impulse, the heart's sounds
(with a double stethoscope), the pulse— its nature and changes
with circumstances, the venous circulation, and many other
THE CIRCULATION OF THE BLOOD,
267
subjects, are all easy of observation, and after a little practice
without liability of causing those aberrations due to the atten-
tion being drawn to one's self.
The observations need not, of course, be confined to the stu-
dent's own person ; it is, however, very important that the nor-
mal should be known before the observer is introduced to cases
of disease. Frequent comparison of the natural and the dis-
eased condition renders physiology, pathology, and clinical
medicine much good service. We again urge upon the student
to try to form increasingly vivid and correct mental pictures of
the circulation under its many changes.
Comparative,— An interesting arrangement of blood-vessels,
known as a rete mirabile, occurs in every main group of verte-
Fig 221 —Rete mirabile of sheep, seen in profile (after Chauveau). The larger rete is
in connection with the encephalic arteries; the smaller, the ophthalmic. The large
artery is the carotid.
brates. An artery breaks up into a great number of vessels of
nearly the same size, which terminate, abruptly and without
capillaries, in another arterial trunk.
They are found in a variety of situations, as on the carotid
and vertebrate arteries of animals that naturally feed from the
ground for long periods together, as the ruminants ; in the
sloth, that hangs from trees ; in the legs of swans, geese, etc. ;
in the horse's foot, in which the arteries break up into many
small divisions. It has been suggested that these arrangements
268 COMPARATIVE PHYSIOLOGY.
permit of a supply of arterial blood being maintained without
congestion of the parts. Very marked tortuosity of vessels, as
in the seal, the carotid of which is said to be forty times as long
Fig. 222. — Section of a lymphatic rete mirabile, from the popliteal space (after Chan-
veau). a, a, afferent vessels, b, b, efferent vessels. The whole very strongly sug-
gests a crude form of lymphatic gland.
as the space it traverses, in all probability serves the same pur-
pose.
Evolution. — The comparative sketch we have given of the
vascular system will doubtless suggest a gradual evolution. We
observe throughout a dependence and resemblance which we
think can not be otherwise explained. The similarity of the
fcetal circulation in the mammal to the permanent circula-
tion of lower groups has much meaning. Even in the highest
form of heart the original pulsatile tube is not lost. The great
veins still contract in the mammal ; the sinus venosus is proba-
bly the result of blending and expansion. The later differentia-
tions of the parts of the heart are clearly related to the adapta-
tion to altered surroundings. Such is seen in the fcetal heart
and circulation, and has probably been the determining cause
of the forms which the circulatory organs have assumed.
It is a fact that the part of the heart that survives the long-
est under adverse conditions is that which bears the stamp of
greatest ancestral antiquity. It (the sinus venosus) may not
THE CIRCULATION OF THE BLOOD.
269
be less under nervous control, but it certainly is least dependent
on tbe nervous system, and bas tbe greatest automaticity.
The law of rhythm in organic nature finds some of its most
evident exemplifications in tbe circulation. Most of tbe
rhytbms are com-
pound, one being |illifl|HflJf
blended with or su-
perimposed on an-
other. Even tbe ap-
parent irregularities
of tbe normal heart
are rhythmical, such
as the very marked
slowing and other
changes accompany-
ing expiration, espe-
cially in some ani-
mals.
We trust we have
made it evident that
the greatest allow-
ance must be made
for the animal group,
and some even for
the individual, in es-
timating any one of
the factors of the cir-
culation. We know
a good deal at present of cardiac physiology, but we do not
know a physiology of " the heart " in the sense in which we
understand that term to have been used till recently — i. e., we
are not in a position to state tbe laws that apply to all forms of
heart.
Summary of the Physiology of the Circulation.— In the
mammal the circulatory apparatus forms a closed system con-
sisting of a central pump or heart, arteries, capillaries, and
veins. All the parts of the vascular system are elastic, but this
property is most developed in the arteries.
Since the tissue-lymph is prepared from the blood in the
capillaries, it may be said that the whole circulatory system
exists for these vessels.
As a result of the action of an intermittent pump on elastic
Fig. 283.— Veins of the foot of the horse (after Chau-
veau).
270 COMPARATIVE PHYSIOLOGY.
vessels against peripheral resistance, in consequence of which
the arteries are always kept more than full (distended), the
flow through the capillaries and veins is constant — a very great
advantage, enabling the capillaries to accomplish their work of
feeding the ever-hungry tissues. While physical forces play a
very prominent part in the circulation of the blood, vital ones
must not be ignored. They lie at the foundation of the whole,
here as elsewhere, and must be taken into the account in every
explanation.
As a consequence of the anatomical, physical, and vital char-
acters of the circulatory system, it follows that the velocity of
the blood is greatest in the arteries, least in the capillaries, and
intermediate in the veins.
The veins with their valves, their superficial position and
thinner walls, make up a set of conditions favoring the onflow
of the blood, especially under muscular exercise.
In the mammal the circulatory system, by reason of its con-
nections with the digestive, respiratory, and lymphatic systems,
and in a lesser degree with all parts of the body, especially the
glandular organs, maintains at once the usefulness and the fit-
ness of the blood.
The arterioles, by virtue of their highly developed muscular
coat, are enabled to regulate the blood-supply to every part, in
obedience to the nervous system.
The blood exercises a certain pressure on the walls of all
parts of the vascular system, which is greatest in the heart it-
self, high in the arteries, lower in the capillaries, and lowest in
the veins, in the largest of which it may be less than the atmos-
pheric pressure, or negative. The heart in the mammal consists
of four perfectly separated chambers, each upper and each
lower pair working synchronously, intermixture of arterial
and venous blood being prevented by septa and interference in
working by valves. The heart is a force-pump chiefly, but, to
some extent, a suction-pump also, though its power as such
purely from its own action and independent of the respiratory
movements of the chest is slight under ordinary circumstances.
In consequence of the lesser resistance in the pulmonary divis-
ion of the circulation, the blood-pressure within the heart is
much less in the right than in the left ventricle — a fact in har-
mony with and causative of the greater thickness of the walls
of the latter; for in the foetus, in which the conditions are dif-
ferent, this distinction does not hold.
THE CIRCULATION OF THE BLOOD. 271
The ventricles usually completely empty themselves of
blood and maintain their systolic contraction even after this
has been effected. The contraction of the heart, which really
begins in the great veins near their junction with the auri-
cles (that do not fully empty themselves), is at once fol-
lowed up by the auricular and ventricular contraction, the
whole constituting one long peristaltic wave. Then follows
the cardiac pause, which is of longer duration than the entire
systole.
When the heart contracts it hardens, owing to closing on a
non-compressible fluid dammed back within its walls by resist-
ance a f route. At the same time the hand placed on the chest-
walls over the heart is sensible of the cardiac impulse, owing
to what has just been mentioned. The systole of the chambers
of the heart gives rise to a first and a second sound, so called,
caused by several events combined, in which, however, the ten-
sion of the valves must take a prominent share. The work of
the heart is dependent on the quantity of blood it ejects and
the pressure against which it acts. The pulse is an elevation
of the arterial wall, occurring with each heart-beat, in conse-
quence of the passage of a wave over the general blood- stream.
There is a distention of the entire arterial system in every direc-
tion. The pulse travels with extreme velocity as compared with
the blood-current. The heart-beat varies in force, frequency,
duration, etc., and with age, sex, posture, and numerous other
circumstances.
The whole of the circulatory system is regulated by the cen-
tral nervous system through nerves. There is in the medulla
oblongata a small collection of nerve-cells making up the
cardio-inhibitory center. This center, with varying degrees of
constancy, depending on the group of animals and the needs
of the organism, sends forth impulses (which modify the beat
of the heart in force and frequency) through the vagi nerves.
There are nerves of the sympathetic system with a center in
the cervical spinal cord, and possibly another in the medulla,
which are capable of originating either an acceleration of the
heart rhythm or an increase of the force of the beat, or both
together, known as accelerators or augmentors. In the verte-
brates thus far examined the vagus is in reality a vago-sympa-
thetic nerve, containing inhibitory fibers proper, and sympa-
thetic, accelerator, or motor fibers.
The inhibitory fibers can arrest, slow, or weaken the cardiac
272 COMPARATIVE PHYSIOLOGY.
beat ; the sympathetic accelerate it or augment its force. When
both are stimulated together, the inhibitory prevail.
These nerves, as also the accelerators, exercise a profound
influence upon the nutrition of the heart, and affect its electri-
cal condition when stimulated, and we may believe when influ-
enced by their own centers.
The inhibitory fibers tend to preserve and restore cardiac
energy ; the sympathetic, whether in the vagus or as the aug-
mentors, the reverse. The vagus nerve (and probably the de-
pressor) acts as an afferent, cardiac sensory nerve reporting on
the intra-cardiac pressure, etc., and so enabling the vaso-
motor and cardio-inhibitory centers, which are, it would seem,
capable of related and harmonious action to act for the general
good.
The arterioles must be conceived as undergoing very fre-
quent changes of caliber. They are governed by the vaso-motor
center, situated in the medulla, and possibly certain subordi-
nate centers in the spinal cord, through vaso-motor nerves.
These are (a) vaso-constrictors, which maintain a constant but
variable degree of contraction of the muscle-cells of the vessels ;
(b) vaso-dilators, which are not in constant functional activity ;
and (c) mixed nerves, with both kinds. An inherited tendency
to rhythmical contraction throughout the entire vascular sys-
tem, including the vessels, must be taken into account.
The depressor nerve acts by lessening the tonic contraction
of (dilating) the vessels of the splanchnic area especially.
It is important to remember that all the changes of the
vascular system, so long as the nervous system is intact — i. e.,
so long as an animal is normal— are correlated ; and that the
action of such nerves as the depressor is to be taken rather as
an example of how some of these changes are brought about,
mere chapters in an incomplete but voluminous history, if we
could but write it all. The changes in blood-pressure, by the
addition or removal of a considerable quantity of blood, are
slight, owing to the sort of adaptation referred to above, effected
through the nervous system. Finally, the capillary circulation,
when studied microscopically, and especially in disordered con-
ditions, shows clearly that the vital properties of these vessels
have an important share in determining the character of the
circulation in themselves directly and elsewhere indirectly.
The study of the circulation , in other groups shows that
below birds the arterial and venous blood undergoes mixture
THE CIRCULATION OF THE BLOOD. 273
somewhere, usually in the heart, but that in all the vertebrates
the best blood is invariably that which passes to the head and
upper regions of the body. The deficiencies in the heart, owing
to the imperfections of valves, septa, etc., are in part counter-
acted in some groups by pressure relations, the blood always
flowing in the direction of least resistance, so that the above-
mentioned result is achieved.
Capillaries are wanting in most of the invertebrates, the
• blood flowing from the arteries into spaces (sinuses) in the tis-
sues. It is to be noted that a modified blood (lymph) is also
found in the interspaces of the cells of organs. Indeed, the
circulatory system of lower forms is in many respects analogous
to the lymphatic system of higher ones.
18
DIGESTION OF FOOD.
The processes of digestion may be considered as having for
their end the preparation of food for entrance into the blood.
This is in part attained when the insoluble parts have been
rendered soluble. At this stage it becomes necessary to inquire
as to what constitutes food or a food.
Inasmuch as animals, unlike plants, derive none of their
food from the atmosphere, it is manifest that what they take in
by the mouth must contain every chemical element, in some
form, that enters into the composition of the body.
But actual experience demonstrates that the food of animals
must, if we except certain salts and water, be in organized
form — i. e., it must approximate to the condition of the tissues
of the body in a large degree. Plants, in fact, are necessary to
animals in working up the elements of the earth and air into
form suitable for them.
Foodstuffs are divisible into :
I. Organic.
1. Nitrogenous, (a) Albumins; (b) Albuminoids (as gelatin).
2. Non-nitrogenous, (a) Carbohydrates (sugars, starches) ;
(b) Fats.
II. Inorganic.
1. Water.
2. Salts.
Animals may derive the whole of their food from the bodies
of other animals (carnivora) ; from vegetable matter exclusively
(herbivora) ; or from a mixture of the animal and vegetable, as
in the case of the pig, bear, and man himself (omnivora).
It has been found by feeding experiments, carried out mostly
on dogs, that animals die when they lack any one of the con-
stituents of food, though they live longer on the nitrogenous
than any other kind. In some instances, as when fed on gela-
tin and water, or sugar and water, the animals died almost as
DIGESTION OP FOOD.
275
soon as if they had been wholly deprived of food. But it has
also been observed that some animals will all but starve rather
than eat certain kinds of food, though chemically sufficient.
We must thus recognize something more in an animal than
merely the mechanical and chemical processes which suffice to
accomplish digestion in the laboratory. A food must be not
only sufficient from the chemical and physical point of view,
but be capable of being acted on by the digestive juices, and of
such a nature as to suit the particular animal that eats it.
To illustrate, bones may be masticated and readily digested
by a hyena, but not by an ox or by man, though they meet the
conditions of a food in containing all the requisite constituents.
Further, the food that one man digests readily is scarcely di-
gestible at all by another ; and it is within the experience of
every one that a frequent change of diet is absolutely necessary.
Since all mammals, for a considerable period of their exist-
ence, feed upon milk exclusively, this must represent a perfect
or typical food. It will be worth while to examine the compo-
sition of milk. The various substances composing it, and their
relative proportions for different animals, may be seen from the
following table, which is based on a total of 1,000 parts :
CONSTITUENTS.
Human.
Cow. Goat.
Ass.
Water
889-08
857-05 863-58
910-24
Casein
j- 39-24
26-66
43-64
1-38
j 48-28 33-60
} 5-76 12-99
4305 43-57
40-37 40-04
5-48 622
j- 20-18
12-56
Albumin
Butter
Milk-sugar
Salts
j- 57-02
Total solids
110-92
142 95 136-42
89 76
The following table, giving the percentage composition of
the milk of different animals, may prove instructive.
Woman.
Cow.
Mare.
Bitch.
Pats
2-00
2-75
0-25
5-00
4-00
4-00
0-60
4-40
2-50
2-00
0-50
5-00
10-00
10-00
Salts
Sugar
0-50
3-50
Total solids
10-00
90-00
13-00
87-00
10-00
90-00
24-00
Water
76-00
276 COMPARATIVE PHYSIOLOGY.
1. The proteids of milk are :
(a.) An albumin very like serum-albumin.
(p.) Casein, normally in suspension, in the form of extremely
minute particles, "which contributes to the opacity of milk.
It can be removed by filtration through porcelain ; and pre-
cipitated or coagulated by acids and by rennet, an extract of
the mucus membrane of the calf's stomach. After this coagu-
lation, whey, a fluid more or less clear, separates, which con-
tains the salts and sugar of milk and most of the water. Much
of the fat is entangled with the casein.
Casein, with some fat, makes up the greater part of cheese.
2. Fats. — Milk is an emulsion — i. e., contains fat suspended
in a fine state of division. The globules, which vary greatly in
size, are surrounded by an envelope of proteid matter. This
covering is broken up by churning, allowing the fatty globules
to run together and form butter.
Butter consists chiefly of olein, palmitin, and stearin, but
contains in smaller quantity a variety of other fats. The ran-
cidity of butter is due to the presence of free fatty acids, espe-
cially butyric.
The fat of milk usually rises to the surface as cream when
milk is allowed to stand.
3. Milk-sugar, which is converted into lactic acid, probably
by the agency of some form of micro-organism, thus furnish-
ing acid sufficient to cause the precipitation or coagulation of
the casein.
Milk, when fresh, should be neutral or faintly alkaline.
4. Salts (and other extractives), consisting of phosphates of
calcium, potassium, and magnesium, potassium chloride, with
traces of iron and other substances.
It can be readily understood why animals fed on milk
rarely suffer from that deficiency of calcium salts in the bones
leading to rickets, so common in the ill-fed. It thus appears
that milk contains all the constituents requisite for the building
up of the healthy mammalian body; and experiments prove
that these exist in proper proportions and in a readily digestible
form. The author has found that a lai'ge number of animals,
into the usual food of which, in the adult form, milk does not
enter, like most of our wild mammals, as well as most birds,
will not only take milk but soon learn to like it, and thrive well
upon it. Since the embryo chick lives upon the egg, it might
have been supposed that eggs would form excellent food for
DIGESTION OF FOOD.
277
adult animals, and common experience proves this to be the
case ; while chemical analysis shows that they, like milk, con-
tain all the necessary food constituents. Meat (muscle, with
fat chiefly) is also, of course, a valuable food, abounding in
Animal Foods.
Explanation of the signs.
Proteids. Albuminoids. N-free org. bodies. Salts.
I2 M^jwsraiiiiii
55
73.5
1'
I"
■»..
0.4
Vegetable Foods.
Explanation of the signs.
Tl'heaten-bread.
Peas.
Rice.
Potatoes.
Wliite Turnip.
Cauliflower.
Beer.
41.3
13
Digestible Non-digestible
N-free organ bodies.
90.5
90
Fig. 234 (Landois).
Salts.
Illllllllllllllllllll
1»
2.5
i-lllllllill
0.5:
I
0.2
10.5.
I1
I0,5
proteids. Cereals contain starch in large proportion, but also a
mixture of proteids. Green vegetables contain little actual nu-
278
COMPARATIVE PHYSIOLOGY.
tritive material, but are useful in furnishing salts and special
substances, as certain compounds of sulphur which, in some ill-
understood way, act beneficially on the metabolism of the body.
They also seem to stimulate the flow of healthy digestive fluids.
Condiments act chiefly, perhaps, in the latter way. Tea, coffee,
etc., contain alkaloids, which it is likely have a conservative
effect on tissue waste, but we really know very little as to how
it is that they prove so beneficial. Though they are recognized
to have a powerful effect on the nervous system as stimulants,
nevertheless it would be erroneous to suppose that their action
was confined to this alone.
The accompanying diagrams will serve to represent to the
eye the relative proportions of the food-essentials in various
articles of diet.
Pig. 225. — Alimentary canal of embryo while the rudimentary mid-gut is still in con-
tinuity with yelk-sac (KQlliker, after Bischoff). A. View from before, a, pharyn-
geal plates; b, pharynx; c, c, diverticula forming the lungs; d, stomach; /, diver-
ticula of liver; g, membrane torn from yelk-sac; A, hind gut. B. Longitudinal
section, a, diverticulum of a lung; b, stomach; c, liver; d, yelk-sac.
It is plain that if, in the digestive tract, foods are changed
in solubility and actual chemical constitution, this must have
been brought about by chemical agencies. That food is broken
up at the very commencement of the alimentary tract is a
matter of common observation; and that there should be a
gradual movement of the food from one part of the canal to
another, where a different fluid is secreted, would be expected.
As a matter of fact, mechanical and chemical forces play a
DIGESTION OF FOOD.
279
large part in the actual preparation of the food for absorption.
Behind these lie, of course, the vital properties of the glands,
which prepare the active fluids from
the blood, so that a study of diges-
tion naturally divides itself into the
consideration of — 1. The digestive
juices; 2. The secretory processes;
and, 3. The muscular and nervous
mechanism by which the food is
carried from one part of the digest-
ive tract to another, and the waste
matter finally expelled.
Embryological— The alimentary
tract, as we have seen, is formed by
an infolding of the splanchnopleure,
and, according as the growth is
more or less marked, does the canal
become tortuous or remain some-
what straight. The alimentary tract
of a mammal passes through stages
of development which correspond with the permanent form of
other groups of vertebrates, according to a general law of evo-
lution. Inasmuch as the embryonic gut is formed of mesoblast
Fig. 226. — Diagram of alimentary
canal of chick at fourth day
(Foster and Balfour, after GOt-
te). Ig, diverticulum of one
lung; St, stomach; I, liver; p,
pancreas.
Fie. 227.— Position of various parts of alimentary canal at different stages. A. Em-
bryo of five weeks. B. Of eight weeks. C. Of ten weeks (Allen "Thomson). I,
pharynx with the lungs; s, stomach; i, small intestine; i', large intestine; g, geni-
tal duct; u, bladder; cl, cloaca; c, caecum; vi, ductus vitello-intestinalis; si, uro-
genital sinus; v, yelk-sac.
280 COMPARATIVE PHYSIOLOGY.
and hypoblast, it is easy to understand why the developed tract
should so invariaably consist of glandular structures and mus-
cular tissue disposed in a certain regular arrangement. The
fact that all the organs that pour digestive juices into the ali-
mentary tract are outgrowths from it serves to explain why
there should remain a physiological connection with an ana-
tomical isolation. The general resemblance of the epithelium
throughout, even in parts widely separated, also becomes clear,
as well as many other points we can not now refer to in detail,
to one who realizes the significance of the laws of descent (evo-
lution).
Comparative. — Amoeba ingests and digests apparently by
every part of its body ; though exact studies have shown that
it neither accepts nor retains without considerable power of
discrimination ; and it is also possible that some sort of digest-
ive fluid may be secreted from the part of the body with which
the food-particles come in contact. It has been shown, too,
that there are differences in the digestive capacity of closely
allied forms among Infusorians.
The ciliated Infusorians have a permanent mouth, which
may also serve as an anus ; or, there may be an anus, though
usually less distinct from the rest of the body than the mouth.
Among the Ccelenterates intra-cellular digestion is found.
Certain cells of the endoderm (as in Hydra) take up food-parti-
cles Amceba-like, digest them, and thus provide material for
other cells as well as themselves, in a form suitable for assimi-
lation. This is a beginning of that differentiation of function
which is carried so far among the higher vertebrates. But, as
recent investigations have shown, such intra-cellular digestion
exists to some extent in the alimentary canal of the highest
members of the vertebrate group (see page 345).
The means for grasping and triturating food among in-
vertebrates are very complicated and varied, as are also those
adapted for sucking the juices of prey. Examples to hand are
to be found in the crab, crayfish, spider, grasshopper, beetle,
etc., on the one hand, and the butterfly, housefly, leech, etc., on
the other.
Before passing on to higher groups, it will be well to bear
in mind that the digestive organs are to be regarded as the out-
come both of heredity and adaptation to circumstances. We
find parts of the intestine, e. g., retained in some animals in
whose economy they seem to serve little if any good purpose, as
DIGESTION OF FOOD.
281
the vermiform appendix of man. Adaptation has been illus-
trated in the lifetime of a single individual in a remarkable
Fig. 228. — Diagram illustrating arrangement of intestine, nervous system, etc., in com-
mon snail, Helix (after Huxley), m, mouth; t, tooth; od, odontophore; g, gullet;
c, crop; a', stomach; r, rectum; a, anus; r. s, renal sac; h, heart; I, lung (modified
pallia) chamber); n. its external aperture; em. thick edge of mantle united with
sides of body; /, foot; cpg, cerebral, pedal, and parieto-splanchnic ganglia aggre-
gated round gullet.
manner ; thus, a seagull, by being fed on grain, has had its
stomach, naturally thin and soft-walled, converted into a mus-
cular gizzard.
Since digestion is a process in which the mechanical and
chemical are both involved, and the food of animals differs so
widely, great variety in the alimentary tract, both anatomical
and physiological, must be expected. Vegetable food must
usually be eaten in much larger bulk to furnish the needed
elements; hence the great length of intestine habitually found
in herbivorous animals, associated often with a capacious
and chambered stomach, furnishing a larger laboratory in
which Nature may carry on her processes. To illustrate, the
stomach of the ruminants consists of four parts (rumen, reticu-
lum, omasum or psalterium, abomasum). The food when
cropped is immediately swallowed; so that the paunch (rumen)
is a mere storehouse in which it is softened, though but little
changed otherwise ; and it would seem that real gastric digestion
is almost confined to the last division, which may be compared
282
COMPARATIVE PHYSIOLOGY.
to the simple stomach, of the Carnivora or of man ; and, before
the food reaches this region, it has been thoroughly masticated
and mixed with saliva.
The stomach of the horse is small, though the intestine,
Fig. 229.— The viscera of a rabbit, as seen upon simply opening the cavities of the
thorax and abdomen without any further dissection. A, cavity of the thorax,
pleural cavity on either side; fi, diaphragm; C, ventricles of the heart; Z>, auri-
cles; E, pulmonary artery; F, aorta; G. lungs collapsed, and occupying only back
part, of chest; //, lateral portions of pleural membranes; /.cartilage at. the end
of sternum (ensiform cartilage); K, portion of the wall of body left between thorax
and abdomen; «, cut ends of the ribs; L, the liver, in this case lying more to the
left than to the right of the body; M, the stomach, a large part of the greater
curvature beting shown; IV, duodenum; O. small intestine; P, the cfecum, so
largely developed in this and other herbivorous animals; Q, the large intestine.
'Huxley.)
DIGESTION OF FOOD.
283
especially the lai'ge gut is capacious. The stomach is divisible
into a cardiac region, of a light color internally, and lined
with epithelium, like that of the oesophagus, and a redder
Pig. 230.
Fig. 232.
Fig. 230. — General and lateral view of dog's teeth (after Chauveau).
Fig. 231.— Anterior view of incisors and canine teeth in a year-old dog (Chauveau).
Fig. 232.— Dentition of inferior jaw of horse (after Chauveau).
pyloric area, in which the greater part of the digestive process
goes on (Fig. 266).
284
COMPARATIVE PHYSIOLOGY.
The mouth parts, even in some of the higher vertebrates, as
the Carnivora, serve a prehensile rather than a digestive pur-
pose. This is well seen in the dog, that bolts his food ; but
in this and allied groups of mammals gastric digestion is very
active.
Tbe teeth as triturating organs find their highest develop-
ment in ruminants, the combined side-to-side and forward-and-
backward motion of the jaws rendering them very effective.
In Carnivora the teeth serve for grasping and tearing, while
in the Insectivora the tongue, as also in certain birds (wood-
peckers), is an important organ for securing food.
Fig. 333. — Profile of upper teeth of the horse, more especially intended to show the
molars, the fangs having been exposed (Chauveau). a, molar teeth; b, supple-
mentary molar; c, tusk; d. incisors.
It is to be noted, too, that, while the horse crops grass by
biting it off, the ox uses the tongue, as well as the teeth and
lips, to secure the mouthful.
Man's teeth are somewhat intermediate in form between the
carnivorous and the herbivorous type. Birds lack teeth, but
the strong muscular gizzard suffices to grind the food against
the small pebbles that are habitually swallowed.
The crop, well developed in granivorous birds, is a dilatation
of the oesophagus, serving to store and soften the food.
In the pigeon a glandular epithelium in the crop secretes a
Fig. 234.— General view of digestive apparatus of fowl (after Chauveau). 1, tongue;
2 pharynx; 3, first portion of oesophagus; 4, crop; 5, second portion of oesopha-
gus; 6, succentric ventricle (proventricuniB); 7, gizzard; 8, origin of duodenum; !),
first branch of duodenal flexure; 10, second branch of same; 11, origin of floating
portion of small intestine; 12, small intestine; 12', terminal portion of this intes-
tine, Hanked on each side by the two coeca (regarded as the analogue of colon of
mammals); 18, 13, free extremities of csecums; 11. insertion of these two culs-de-
sac into intestinal tube; 15, rectum; lfi, cloaca; 17', anus; 18, mesentery; 19. left
lobe ol' liver; 20, right, lobe; 21, gall-bladder; 22, insertion of pancreatic and biliary
ducts; the two pancreatic ducts are the most anterior, the choledic or hepatic is in
the middle, and the cystic duct is posterior; 23, pancreas; 24, diaphragmatic aspect
of lung; 25, ovary (in a state of atrophy); 2G, oviduct.
Pia. 234.
286
COMPARATIVE PHYSIOLOGY.
milky-looking substance that is regurgitated into the mouth of
the young one, which is inserted within that of the parent bird.
The proventriculus — an enlargement just above the gizzard
—is relatively to the latter very thin -walled, but provides the
true gastric juices.
Certain plants digest proteid matter, like animals ; thus the
sun-dew (Drosera), by the closure of its leaves, captures insects,
which are digested and the products absorbed. The digestive
fluid consists of a pepsin-containing secretion, together with
formic acid.
STRUCTURE, ARRANGEMENT, AND SIGNIFICANCE
OF THE TEETH.
In a tooth we recognize a portion imbedded in the jaw (fang,
root), a free portion (crown), and a constricted region (neck).
B
Ki... ;r,:,
Fig. 237.
Pia
835— Magnified section of a canine tooth, showing its intimate structure. 1,
crown- 2 -i neck- 3 fang, or root; 4, cavitas pulpae; 5, opening by which the ves-
sels and nerves communicate with the pulp; 6, 6, ivory, showing fibrous structure;
7,7, enamel; 8, 8, cement. , .
Pio. 236.— A, transverse section of enamel, Showing its hexagonal prisms; B, sepa-
rated prisms (Cliauveau).
Kir. !.':{7 Section through fang of molar tooth (Cliauveau). a, a. dentine traversed
by its tubuli; b, b, interglobular or nodular layer; c, c, cementum.
DIGESTION OF POOD. 287
Pig. 238.— Incisor teeth of the horse. Details of structure (Chauveau). 1, a tooth in
which is indicated general shape of a permanent incisor and the particular forms
successively assumed by dental table in consequence of friction and the continued
pushing outward of these teeth; 2. a virgin tooth, anterior and posterior faces; 3,
longitudinal section of a virgin tooth, intended to show the internal conformation
and' structure. Not to complicate the figure, the external cement and that amassed
in the infundibulum have not been exhibited; 4, transverse section for the same
purpose; a, encircling enamel; b, central enamel; c, dental star; d, dentine; 5, de-
ciduous tooth.
A tooth is made up of enamel, dentine or " ivory," and ce-
ment (crusta petrosa). The relative distribution of these is
shown in Fig. 235.
B D
Fig. 239.— Transverse section of a horse's upper molar tooth (Chauveau). A, external
cement; JB, external enamel; C, dentine; D, internal enamel; E, internal cement.
288
COMPARATIVE PHYSIOLOGY.
Enamel is made up of elongated hexagonal prisms set almost
vertically in the dentine (Fig. 236).
It is the hardest substance known in the animal body, con-
sisting almost entirely of inorganic material ; and when lost is
but indifferently if at all replaced.
Fio. 240.— Tooth of cat in situ (Waldeyer). 1, enamel; 2, dentine; 3, cement; 4, peri-
osteum of alveolar cavity; 5, bone of jaw; 6, pulp cavity.
Dentine is traversed by the dentine tubules (Fig. 237),
which radiate outward from the pulp cavity.
DIGESTION OF FOOD.
289
The latter is filled by the tooth-pulp, which consists of a
delicate connective tissue supporting blood-vessels and nerves
which ramify in it after entering by the openings in the fang
of the tooth. From the pulp protoplasmic fibers extend into
the dentine tubules.
The crusta petrosa is very similar to bone, but is usually
without Haversian canals, and, like bone, is covered with peri-
osteum.
Fig. 241.— The teeth of the ox (Chauveau). 1, upper jaw, with a, friction surface and
b, external surface; 2, lower jaw, with 0, dental tables, and b, external face. '
Teeth are simple and compound. In the former (carnivora)
the entire crown is covered with enamel ; in the latter, owing
19
290
COMPARATIVE PHYSIOLOGY.
to wear, the other constituents appear on the upper surface of
the crown (Figs. 238, 239, 240).
It follows that the former are better adapted for tearing,
the latter for grinding, as the different components wear un-
equally and leave the top of the crown rough, so that the upper
and lower jaws of a ruminant are like two millstones, (Fig.
241).
It also follows that in the horse and in ruminants the age
may be learned with considerable accuracy from the condition
of wear of the teeth and as the incisors are most readily ex-
amined they are taken as the chief indicators of the age of
the animal.
In nearly all animals are found the deciduous or milk teeth
succeeded by the permanent teeth. This arises as a necessity
from the growth of the jaw and the need of stronger teeth, either
as weapons of defense and attack or in order the more effectu-
ally to secure and prepare food. The permanent teeth are also
more numerous than the milk teeth.
The dentition of our domestic animals may be expressed
thus :
Dos.
Cat.
Man.
Pig-
Ox.
Horse.
. 3-3
Incisors, o~~q
1-1
canines, y~. r
3-3
1-1
3-3
1-1
2-2
1-1
2-2
1-1
3-3
1-1
3-3
1-1
0-0
0-0
3-3
1-1
3-3
1-1
3-3
1-1
3-3
0-0
3-3
0-0
4-4
premolars, t^7
3-3
2-2
2-2
2-2
3-3
3-3
3-3
3-3
3-3
3-3
0-0
0-0
2-2
molars, 5—5 = 42.
1-1
1-1
3-3
3-3
4-4
4-4
3-3
3-3
3-3
3-3
3-3
3-3
= 30.
= 32.
= 44.
32.
= 40.
= 24.
The latter is the representation of the milk dentition. The
mare is without canines ("tushes").
It will be noticed that in the ox incisors and canines do not
appear in the upper jaw, though they are represented by embry-
onic rudiments.
The table above and that on page 296 (after Leyh) give a
large amount of information in a small space, and are illus-
trated by the accompanying figures :
DIGESTION OF FOOD.
291
Fig. 242.— The teeth of the pig (Chauveau). 1, upper teeth, table surface; 2, lower
teeth, table aspect; 3, lateral view of jaws.
2$ years. 4 years.
(6 broad incisors.) (8 broad incisors.)
Over 7 years,
broad incisors.)
2 months.
(Milk-teeth.)
1± years.
(2 broad incisors.)
If years.
(4 broad incisors.)
Fig. 243. — Changes in incisor teeth of the sheep (WilckensX
292
COMPARATIVE PHYSIOLOGY.
New-born.
3 months.
6 months
JSwSl^
■"■'■"■■ " ■ ' ■
1 year.
2 years.
2£ years.
3 years.
4 years.
5 years.
(i years. 7 years.
Fig. 244 (1).— Changes in incisor teeth of horse with age (Wilckens).
DIGESTION OF POOD
293
8 years.
it years.
12 years.
15 years.
18 years. 24 years.
Fig. 244 (2). Changes in incisor teeth of horse with age (Wilckens).
294
COMPARATIVE PHYSIOLOGY.
1
New-born.
JSD
4 weeks.
1 year.
2 years.
2| years.
3 years.
•3J years.
4 years. 5 years.
Fig. 245 (1).— Changes in incisor teeth of ox with age (Wilckens).
DIGESTION OF POOD.
295
6 years.
' years.
8 years.
10 years.
12 years.
14 years.
16 years. 18 years. 20 years.
Fig. 245 (2).— Changes in incisor teeth of ox with age (Wilckens).
296
COMPARATIVE PHYSIOLOGY.
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«5 53
DIGESTION OP FOOD. 297
THE DIGESTIVE JUICES.
Saliva. — The saliva as found in the mouth is a mixture of
the secretion of three pairs of glands, alkaline in reaction, of a
low specific gravity (variable in different groups of animals),
with a small percentage of solids consisting of salts and organic
bodies (mucin, proteids).
Saliva serves mechanical functions in articulation, in moist-
ening the food, and dissolving out some of its salts. But its
principal use in digestion is in reducing starchy matters to a
soluble form, as sugar. So far as known, the other constituents
of the food are not changed chemically in the mouth.
The Amylolytic Action of the Saliva.— Starch exists in grains
surrounded by a cellulose covering, which saliva does not digest :
hence its action on raw starch is slow.
It is found that if a specimen of boiled starch not too thick
be exposed to a small quantity of saliva at the temperature of
the body or thereabout (37° to 40° C), it will speedily undergo
certain changes :
1. After a very short time sugar may be detected by Feh-
ling's solution (copper sulphate in an excess of sodium hydrate,
the sugar reducing the cupric hydrate to cuprous oxide on
boiling).
2. At this early stage starch may still be detected by the
blue color it gives with iodine ; but later, instead of a blue, a
purple or red may appear, indicating the presence of dextrin,
which may be regarded as" a product intermediate between
starch and sugar.
3. The longer the process continues, the more sugar and the
less starch or dextrin to be detected ; but, inasmuch as the
quantity of sugar at the end of the process does not exactly
correspond with the original quantity of starch, even when no
starch or dextrin is to be found, it is believed that other bodies
are formed. One of these is achroodextrin, which does not
give a color reaction with iodine.
The sugars formed are: (a) Dextrose. (6) Maltose, which
has less reducing power over solutions of copper salts, a more
pronounced rotatory action on light, etc.
It is found that the digestive action of saliva, as in the
above-described experiment, will be retarded or arrested if the
sugar is allowed to accumulate in large quantity. That diges-
tion in the mouth is substantially the same as that just de-
298 COMPARATIVE PHYSIOLOGY.
scribed can be easily sbown by holding a solution of starch in
the mouth for a few seconds, and then testing it for sugar,
when it will be invariably found.
While salivary digestion is not impossible in a neutral me-
dium, it is arrested in an acid one even of no great strength
(less than one per cent), and goes on best in a feebly alkaline
medium, which is the condition normally in the mouth. Though
a temperature about equal to that of the body is best adapted
for salivary digestion, it will proceed, we have ourselves found
at a higher temperature than digestion by any other of the
juices, so far as man is concerned — a fact to be connected, in all
probability, with his habit for ages of taking very warm fluids
into the mouth.
The active principle of saliva is ptyalin, a nitrogenous body
which is assumed to exist, for it has never been perfectly iso-
lated. It belongs to the class of unorganized ferments, the
properties of which have been already referred to before (page
162).
Characteristics of the Secretion of the Different Glands —
Parotid saliva is in man not a viscid fluid, but clear and limpid,
containing very little mucin. Submaxillary saliva in most
animals and in man is viscid, while the secretion of the sub-
lingual gland is still more viscid.
Comparative. — Saliva differs greatly in activity in different
animals ; thus saliva in the dog is almost inert, that of the
parotid gland quite so ; in the cat it is but little more effective ;
and in the horse, ox, and sheep, it is known to be of very feeble
digestive power.
In man, the Guinea-pig, the rat, the hog, both parotid and
submaxillary saliva are active ; while in the rabbit the sub-
maxillary saliva, the reverse of the preceding, is almost in-
active, and the parotid secretion very powerful.
An aqueous or glycerin extract of the salivary glands has
digestive properties. The secretion of the different glands may
be collected by passing tubes or cannulas into their ducts.
The saliva, normally neutral or only faintly acid, may be-
come very much so in the intervals of digestion. The rapid
decay of the teeth occurring during and after certain diseases
seems in certain cases to be referable in part to an abnormal
condition of the saliva.
The tartar which collects on the teeth consists largely of
earthy phosphates.
DIGESTION OP FOOD. 299
Gastric Juice. — Gastric juice may be obtained from a fistu-
lous opening into the stomach. Such may be made artificially
by an incision over the organ in the middle line, catching it up
and stitching it to the edges of the wound, incising and insert-
ing a special form of cannula, which may be closed or opened
at will.
Digestion in a few cases of accidental gastric fistulas has
been made the subject of careful study. The most instructive
case is that of Alexis St. Martin, a French Canadian, into
whose stomach a considerable opening was made by a gunshot-
wound.
Gastric juice, in his case and in the lower animals with
artificial openings in the stomach, has been obtained by irri-
tating the mucous lining mechanically with a foreign body, as
a feather.
The great difficulty in all such cases ai^ises from the impos-
sibility of being certain that such fluid is normal ; for the con-
ditions which call forth secretion are certainly such as the
stomach never experiences in the ordinary course of events,
and we have seen how saliva vaiies, according as the animal is
fasting or feeding, etc.
Bearing in mind, then, that our knowledge is possibly only
approximately correct, we may state what is known of the se-
cretions of the stomach.
The gastric secretion is clear, colorless, of low specific grav-
ity (lOOl to 1010), the solids being in great part made up of
pepsin with a small quantity of mucus, which may become ex-
cessive in disordered conditions. There has been a good deal
of dispute as to the acid found in the stomach during digestion.
It is now generally agreed that during the greater part of the
digestive process there is free hydrochloric acid to the extent
of about "2 per cent. It is maintained that lactic acid exists
normally in the early stages of digestion, and it is conceded that
lactic, butyric, acetic, and other acids may be present in certain
forms of disordered digestion.
It is also generally acknowledged that in mammals the work
of the stomach is limited, so far as actual chemical changes go,
to the conversion of the proteid constituents of food into pep-
tone. Fats maybe released from their proteid coverings (cells),
but neither they nor starches are in the least altered chemically.
Some have thought that in the dog there is a slight digestion of
fats in the stomach. The solvent power of the gastric juice is
300 COMPARATIVE PHYSIOLOGY.
greater than can be accounted for by the presence of the acid it
contains merely, and it has a marked antiseptic action.
Digestive processes may be conducted out of the body in a
very simple manner, which the student may carry out for him-
self. To illustrate by the case of gastric digestion : The mucous
membrane is to be removed from a pig's stomach after its sur-
face has been washed clean, but not too thoroughly, chopped
up fine, and divided into two parts. On one half pour water
that shall contain "2 per cent hydrochloric acid (made by add-
ing 4 to 6 cc. commercial acid to 1,000 cc. water). This will
extract the pepsin, and may be used as the menstruum in which
the substance to be digested is placed. The best is fresh fibrin
whipped from blood recently shed.
Since the fluid thus prepared will contain traces of peptone
from the digestion of the mucous membrane, it is in some
respects better to use a glycerin extract of the same. This is
made by adding some of the best glycerin to the chopped up
mucous membrane of the stomach of a pig, etc., well dried with
bibulous paper, letting the whole stand for eight to ten days,
filtering through cotton, and then through coarse filter-paper.
It will be nearly colorless, clear, and powerful, a few drops
sufficing for the work of digesting a little fibrin when added to
some two per cent of hydrochloric acid.
Digestion goes on best at about 40° C, but will proceed in
the cold if the tube in which the materials have been placed is
frequently shaken. It is best to place the test-tube containing
them in a beaker of water kept at about blood-heat. Soon the
fibrin begins to swell and also to melt away.
After fifteen to twenty minutes, if a little of the fluid in the
tube be removed and filtered, and to the filtrate added carefully
to neutralization dilute alkali, a precipitate, insoluble in water
but soluble in excess of alkali (or acid), is thrown down. This
is in most respects like acid-albumen, but has been called para-
peptone. The longer digestion proceeds, the less is there of
this and the more of another substance, peptone, so that the
former is to be regarded as an intermediate product. Peptone
is distinguished from albuminous bodies or proteids by — 1.
Not being coagtilable from its aqueous solutions on boiling.
2. Diffusing more readily through animal membranes. 3. Not
being precipitated by a number of reagents that usually act
on proteids.
In artificial digestion it is noticeable that much more fibrin
DIGESTION OP FOOD.
301
or other proteid matter will be dissolved if it be finely divided
and frequently shaken up, so that a greater surface is exposed
to the digestive fluid.
The exact nature of the process by which proteid is changed
to peptone is not certainly known.
Since starch on the addition of water becomes sugar (C6Hi0
05 + H20 = CoHjoOo), and since peptones have been formed
through the action of dilute acid at a high temperature or by
superheated water alone, it is possible that the digestion of both
starch and proteids may be a hydration ; but we do not know
that it is such.
As already explained, milk is curdled by an extract of the
stomach (rennet) ; and this can take place in the absence of all
acids or anything else that migbt be suspected except the real
cause ; tbere seems to be no doubt that there is a distinct fer-
ment which - produces the coagulation of milk which results
from the precipitation of its casein.
The activity of the gastric juice, and all extracts of the mu-
cous membrane of the stomach, on proteids, is due to pepsin, a
nitrogenous body, but not a proteid.
Like other ferments, the conditions under which it is effect-
ive are well defined. It will not act in an alkaline medium at
all, and if kept long in such it is destroyed. In a neutral me-
dium its power is suspended but not destroyed. Digestion will
go on, though less perfectly, in the presence of certain other
acids than hydrochloric. As with all digestive ferments, the
activity of pepsin is wholly destroyed by boiling.
IN 100 PARTS.
Man.
Ox.
Pig.
DOG.
Fresh.
From gall-
bladder.
Water
86-3
13-7
7-4
3-0
2-2
1-1
90-4
. 9-6
[■ 8-0
0-3
1-3
ss-s
11-2
| 7-3
S 2-2
0-6
1-1
95-3
4-7
3-4
0-5
0-2
0-6
85-2
Solids
14-8
Bile salts
12-6
Fats, soaps
1-3
Mucin and coloring matter . .
Inorganic salts
0-3
0-6
The color of the bile of man is a rich goldeu yellow. When
it contains much mucus, as is the case when it remains long in
the gall-bladder, it is ropy, though usually clear. Bile may
contain small quantities of iron, manganese, and copper, the
302 COMPARATIVE PHYSIOLOGY.
latter two especially being absent from all other fluids of the
body. Sodium chloride is the most abundant salt. Bile must
be regarded as an excretion as well as a secretion ; the pig-
ments, copper, manganese, and perhaps the iron and the cho-
lesterin being of little or no use in the digestive processes, so
far as known.
The bile-salts are the essential constituents of bile as a
digestive fluid. In man and many other animals, they consist
of taurocholate and glycocholate of sodium, and may be ob-
tained in bundles of needle-shaped crystals radiating from a
common center. These salts are soluble in water and alcohol,
with an alkaline reaction, but insoluble in ether.
Glycocholic acid may be resolved into cholalic (cholic) acid
and glycin (glycocol) ; and taurocholic acid into cholalic acid
and taurin. Thus :
Glycocholic acid. Cholalic acid. Glycin.
C26H43N08 + H20 = C24H40O5 + C2H*N02.
Taurocholic acid. Cholalic acid. Taurin.
CseBUsNSO, + H20 = C24H40O6 + C2H7NS03.
Glycocol (glycin) is amido-acetic acid
CH2<(
Taurin, amido-isethionic acid,
CH<co!k'and
C2H4<tstt| , and may be made artifi-
cially from isethionic acid.
It is to be noted that both the bile acids contain nitrogen,
but that cholalic acid does not. The decomposition of the bile
acids takes place in the alimentary canal, and the glycin and
taurin are restored to the blood, and are possibly used afresh in
the construction of the bile acids, though this is not definitely
known.
Bile-Pigments. — The yellowish-red color of the bile is owing
to Bilirubin (CioHmNaOa), which may be separated either as
an amorphous yellow powder or in tablets and prisms. It is
soluble in chloroform, insoluble in water, and but partially
soluble in alcohol and ether. It makes up a large part of
gall-stones, which contain, besides cholesterin, earthy salts in
abundance.
It may be oxidized to Biliverdin (CmHisN^), the natural
green pigment of the bile of the herbivora. When a drop of
DIGESTION OP FOOD. 303
nitric acid, containing nitrous acid, is added to bile, it under-
goes a series of color changes in a certain tolerably constant
order, becoming green, greenish-blue, blue, violet, a brick red,
and finally yellow ; though the green is the most characteristic
and permanent. Each one of these represents a distinct stage
of the oxidation of bilirubin, the green answering to biliverdin.
Such is Gmelin's test for bile-pigments, by which they may be
detected in urine or other fluids. The absence of proteids in
bile is to be noted.
The Digestive Action of Bile. — 1. So far as known, its action
on proteids is nil. When bile is added to the products of an
artificial gastric digestion, bile-salts, peptone, pepsin, and para-
peptone are precipitated and redissolved by excess. 2. It is
slightly solvent of fats, though an emulsion made with bile is
very feeble. But it is likely helpful to pancreatic juice, or
more efficient itself when the latter is present. With free fatty
acids it forms soaps, which themselves help in emulsifying fat.
3. Membranes wet with bile allow fats to pass mere readily;
hence it is inferred that bile assists in absorption. 4. When
bile is not poured out into the alimentary canal the faeces
become clay-colored and ill-smelling, foul gases being secreted
in abundance, so that it would seem that bile exercises an anti-
septic influence. It may limit the quantity of indol formed.
It is to be understood that these various properties of bile are
to be traced almost entirely to its salts ; though its alkaline
reaction is favorable to digestion in the intestines, apart from
its helpfulness in soap-forming, etc. 5. It is thought by some
that the bile acts as a stimulant to the intestinal tract, giving
rise to peristaltic movements, and also, mechanically, as a lubri-
cant of the faeces. In the opinion of many, an excess of bile
naturally poured out causes diarrhoea, and it is well known
that bile given by the mouth acts as a purgative. However,
we must distinguish between the action of an excess and that
of the quantity secreted by a healthy individual. The acid of
the stomach has probably no effect allied to that produced by
giving acids medicinally, which warns us that too much must
not be made out of the argument from bilious diarrhsea. 6. As
before intimated, a great part of the bile must be regarded as
excrementitious. It looks as though much of the effete haemo-
globin of the blood and of the cholesterin, which represents
possibly some of the waste of nervous metabolism, were expelled
from the body by the bile. The cholalic acid of the fasces is
304
COMPARATIVE PHYSIOLOGY.
derived from the decomposition of the bile acids. Part of their
mucus must also be referred to the bile, the quantity originally
present in this fluid depending much on the length of its stay
in the gall-bladder, which secretes this substance. 7. There is
throughout the entire alimentary tract a secretion of mucus
which must altogether amount to a large quantity, and it has
been suggested that this has other than lubricating or such like
functions. It appears that mucus may be resolved into a pro-
teid and an animal gum, which latter, it is maintained, like
vegetable gums, assists emulsification of fats. If this be true,
and the bile is, as has been asserted, possessed of the power to
break up this mucus (mucin), its emulsifying effect in the in-
testine may indirectly be considerable. Bile certainly seems to
intensify the emulsifying power of the pancreatic juice.
There does not seem to be any ferment in bile, unless the
power to change starch into sugar, peculiar to this secretion in
some animals, is owing to such.
Comparative. — The bile of the carnivora and omnivora is
yellowish-red in color; that of herbivora green. The former
contains taurocholate salts almost exclusively ; in herbivorous
animals and man there is a mixture of the salts of both acids,
tbough tbe glycocholate predominates.
Fig. 240.— Gallbladder, ductus choledochus and pancreas in man (after Le Bon). _ a,
gall-bladder: l>. hepatic duct; c, opening of second duct of pancreas; <l. opening
of main pancreatic duct and bile-duct; e, e, duodenum; /, ductus choledochus; p,
pancreas.
DIGESTION OF FOOD.
305
Pancreatic Juice. — This fluid is found to vary a good deal
quantitatively, according as it is obtained from a temporary
(freshly made) or permanent fistula — a fact which emphasizes
the necessity for caution in drawing conclusions about the
digestive juices as obtained by our present methods.
The freshest juice obtainable through a recent fistulous
opening in the pancreatic duct is clear, colorless, viscid, alka-
line in reaction, and with a very variable quantity of solids
(two to ten per cent), less than one per cent being inorganic
matter.
Among the organic constituents the principal are albumin,
alkali-albumin, peptone, leucin, tyrosin, fats, and soaps in small
amount. The alkalinity of the juice is owing chiefly to sodium
Fig. 247.— Crystals of leucin (Funke).
Fig. 248.— Crystals of tyrosin (Funke).
carbonates, which seem to be associated with some proteid
body. There is little doubt that leucin, tyrosin, and peptone
arise from digestion of the proteids of the juice by its own
action.
Experimental.— If the pancreatic gland be mostly freed from
adhering fat, cut up, and washed so as to get rid of blood;
then minced as fine as possible, and allowed to stand in one-per-
cent sodium-carbonate solution at a temperature of 40° C, the
following results maybe noted: 1. After a variable time the
reaction may change to acid, owing to free fatty acid from
the decomposition (digestion) of neutral fats. 2. Alkali-albu-
min, or a body closely resembling it, may be detected and sep-
arated by neutralization. 3. Peptone may be detected by the
20
306 COMPARATIVE PHYSIOLOGY.
use of a trace of copper sulphate added to a few drops of caustic
alkali, which becomes red if this body be present. 4. After a
few hours the smell becomes faecal, owing' in part to indol,
which gives a violet color with chlorine-water; while under
the microscope the digesting mass may be seen to be swarming
with bacteria. 5. When digestion has proceeded for some time,
leucin and tyrosin may be shown to be present, though their
satisfactory separation in crystalline form involves somewhat
elaborate details. These changes are owing to self-digestion
of the gland.
All the properties of this secretion may be demonstrated
more satisfactorily by making an aqueous or, better, glycerin
extract of the pancreas of an ox, pig, etc., and carrying on arti-
ficial digestion, as in the case of a peptic digestion, with fibrin.
In the case of the digestion of fat, the emulsifying power of a
watery extract of the gland may be shown by shaking up a
little melted hog's lard, olive-oil (each quite fresh, so as to show
no acid reaction), or soap. Kept under proper conditions, free
acid, the result of decomposition of the neutral fats or soap
into free acid, etc., may be easily shown. The emulsion, though
allowed to stand long, persists, a fact which is availed of to
produce more palatable and easily assimilated preparations of
cod-liver oil, etc., for medicinal use.
Starch is also converted into sugar with great ease. In
short, the digestive juice of the pancreas is the most complex
and complete in its action of the whole series. It is amylolytic,
proteolytic, and steaptic, and these powers have been attributed
to three distinct ferments— amy lopsin, tripsin, and steapsin.
Proteid digestion is carried further than by the gastric juice,
and the quantity of crystalline nitrogenous products formed is
in inverse proportion to the amount of peptone, from which it
seems just to infer that part of the original peptone has been
converted into these bodies, which are found to be abundant or
not in an artificial digestion, according to the length of time
it has lasted— the longer it has been under way the more leucin
and tyrosin present. Leucin is another compound into which
the amido (NH2) group enters to make amido-caproic acid— one
of the fatty series— while tyrosin is a very complex member of
the aromatic series of compounds. Thus complicated are the
chemical effects of the digestive juices ; and it seems highly
probable that these are only some of the compounds into which
the proteid is broken up. Though putrefactive changes with
DIGESTION OF FOOD.
307
formation of indol, etc., occur in pancreatic digestion, both
within and without the body, they are to be regarded as acci-
dental, for by proper precautions digestion may be carried on
et%*>
3Ml
«SB!ao '
1 If '^
ofi, I
Fig. 849.— Micro-organisms of large intestine (after Landois), 1. bacterium coli com-
mune; 2, bacterium lactis aerogenes; 3, 4, large bacilli of Bienstock, with partial
endogenous spore-formation; 5, various stages of development of bacillus which
causes fermentation of albumen.
in the laboratory without their occurrence, and they vary in
degree with the animal, the individual, the food, and other con-
ditions. It is not, however, to be inferred that micro-organisms
serve no useful purpose in the alimentary canal ; the subject,
in fact, requires further investigation.
Succus Entericus.— The difficulties of collecting the secre-
tions of Lieberkuhn's, Briinner's, and other intestinal glands will
be at once apparent. But by dividing the intestine in two
places, so as to isolate a loop of the gut, joining the sundered
Fig. 250.— Portion of one of Briinner's glands (Chauveau).
ends by ligatures, thus making the continuity of the main gut
as complete as before, closing one end of the isolated loop, and
308
COMPARATIVE PHYSIOLOGY.
bringing the other to the exterior, as a fistulous opening, the
secretions could be collected, food introduced, etc.
But it seems highly improbable that information approxi-
mately correct at best, and possibly highly misleading, could
"m
•i)-,y)
Pig. 251.— Intestinal tubules (follicles of Lieberkiihn) 1 x 100 (Sappey). A, from dog;
B, ox; C, sheep; D, pig; E, rabbit.
be obtained in such manner. Moreover, the greatest diversity
of opinion prevails as to the facts themselves, so that it seems
scarcely worth while to state the contradictory conclusions ar-
rived at.
It is, however, on the face of it, probable that the intestine —
even the large intestine — does secrete juices that in herbivora,
at all events, play no unimportant part in the digestion of their
Fig. 252. — General view of horse's intestines; animal is placed on its back, and intes-
tinal mass spread out (after Ohaiiveau). A, duodenum as it passes behind great
mesenteric artery; B, free portion of small intestine; 0, ileocaecal portion; D,
creciim; E, I<\ G, loop formed by large colon; G, pelvic flexure; F, F, point
where colic loop is doubled to constitute suprasternal and diaphragmatic flexures.
Fig. 352.
310 COMPARATIVE PHYSIOLOGY.
bulky food ; and it is also probable, as in so many otber in-
stances, that, when the other parts of the digestive tract fail
when the usual secretions are not prepared or do not act on the
food, glands that normally play a possibly insignificant part
may function excessively — we may almost say vicariously —
and that such glands must be sought in the email intestine.
There are facts in clinical medicine that seem to point strongly
in this direction, though the subject has not yet been reduced to
scientific form.
Comparative.— Within the last few years the study of vege-
table assimilation from the comparative aspect has been fruit-
ful in results which, together with many other facts of vegeta-
ble metabolism, show that even plants ranking high in the
organic plane are not in many of their functions so different
from animals as has been supposed. It has been known for a
longer period that certain plants are carnivorous ; but it was
somewhat of a surprise to find, as has been done within the
past few years, that digestive ferments are widely distributed
in the vegetable kingdom and are found in many different parts
of plants. What purpose they may serve in the vegetable
economy is as yet not well known. At present it would seem
as though, from their presence in so many cases in the seed,
they might have something to do with changing the cruder
forms of nutriment into such as are better adapted for the nour-
ishment of the embryo.
Thus far, then, not only diastase but pepsin, a body with
action similar to trypsin, and a rennet ferment, rank among the
vegetable ferments best known.
A ferment has been extracted from the stem, leaves, and un-
ripe fruit of Carica papaya, found in the East and West Indies
and elsewhere, which has a marked proteolytic action.
It is effective in a neutoal, most so in an alkaline medium ;
and, though its action is suspended in a feeble acid menstruum,
it does not appear to be destroyed under such circumstances, as
is trypsin. This body is attracting a good deal of attention,
and its use has been recently introduced into medical prac-
tice.
Very lately also a vegetable rennet has been found in sev-
eral species of plants. The subject is highly promising and
suggestive.
DIGESTION OF FOOD.
311
SECRETION AS A PHYSIOLOGICAL PROCESS.
Secretion of the Salivary Glands.— We shall treat this subject
at more length because of the light it throws on the nervous
phenomena of vital process ; and, since the salivary glands have
been studied more thoroughly and successfully than any other,
they will receive greater attention.
Fig. 253.
Fig. 254.
Fig. 253.— Lobule of parotid gland, injected with mercury, and magnified 50 diame-
ters.
Fig. 254.— Capillary network around the follicles of the parotid gland.
The main facts, ascertained experimentally and otherwise,
are the following :
Assuming that the student is familiar with the general ana-
tomical relations of the salivary glands in some mammal, we
would further remind him that the submaxillary gland has a
double nervous supply : 1. From the cervical sympathetic by
branches passing to tbe gland along its arteries. 2. From the
chorda tympani nerve, which after leaving the facial makes
connection with the lingual, whence it proceeds to its desti-
nation.
The following facts are of importance as a basis for conclu-
sions: 1. It is a matter of common observation that a flow of
saliva may be excited by the smell, taste, sight, or even thought
of food. 2. It is also a matter of experience that emotions, as
fear, anxiety, etc., may parch the mouth — i. e., ari'est the flow
of saliva. The excited speaker thus suffers in his early efforts.
3. If a glass tube be placed in the duct of the gland and any
substance that naturally causes a flow of saliva be placed on
the tongue, saliva may be seen to rise rapidly in the tube. 4.
The same may be observed if the lingual nerve, the glossopha-
312
COMPARATIVE PHYSIOLOGY.
ryngeal, and many other nerves be stimulated ; also if food be
introduced into the stomach through a fistula. 5. If the pe-
Fig. 255.— Maxillary and sublingual gland (Chauveau). R, maxillary gland; S, Whar-
ton's duct; T, sublingual gland.
ripheral end of the chorda tympani be stimulated, two results
follow : (a) There is an abundant flow of saliva, and (6) the
arterioles of the gland become dilated ; the blood may pass
through with such rapidity that the venous blood may be
bright red in color and there may be a venous pulse. 7. Stimu-
lation of the medulla oblongata gives rise to a flow of saliva,
which is not possible when the nerves of the gland, especially
the chorda tympani, are divided; nor can a flow be then excited
by any sort of nervous stimulation, excepting that of the ter-
minal branches of the nerves of the gland itself. 8. If the sym-
pathetic nerves of the gland be divided, there is no immediate
flow of saliva, though there may be some dilatation of its ves-
DIGESTION OF FOOD.
313
sels. 9. Stimulation of the terminal ends of the sympathetic
and chorda nerves causes a flow of saliva, differing as to total
quantity and the amount of contained solids; hut the nerve
that produces the more abundant watery secretion, or the re-
verse, varies with the animal, e. g., in the cat chorda saliva is
Part of brain above medulla
Afferent nerves
from tongue-
Fig. 256. -Diagram intended to indicate the nervous mechanism of salivary secretion.
more viscid, in the dog less so ; though in all animals as yet
examined it seems that the secretion as a result of stimulation
of the chorda tympani nerve is the most abundant ; and in the
314 COMPARATIVE PHYSIOLOGY.
case of stimulation of the chorda the vessels of the gland are
dilated, while in the case of the sympathetic they are con-
stricted. 10. If atropin be injected into the blood, it is impos-
sible to induce salivary secretion by any form of stimulation,
though excitation of the chorda nerve still causes arterial dila-
tation.
Conclusions. — 1. There is a center in the medulla presiding
over salivary secretion. 2. The influence of this center is ren-
dered effective through the chorda tympani nerve at all events,
if not also by the sympathetic. 3. The chorda tympani nerve
contains both secretory and vaso-dilator fibers; the sympathetic
secretory and vaso-constrictor fibers. 4. Arterial change is not
essential to secretion, though doubtless it usually accompanies
it. Secretion may be induced in the glands of an animal after
decapitation by stimulation of its chorda tympani nerve, analo-
gous to the secretion of sweat in the foot of a recently dead
animal, under stimulation of the sciatic nerve. 5. The char-
acter of the saliva secreted varies with the nerve stimulated, so
that it seems likely that the nervous centers normally in the
intact animal regulate the quality of the saliva through the
degree to which one or the other kind of nerves is called into
action. 6. Secretion of saliva may be induced reflexly by ex-
periment, and such is probably the normal course of events.
7. The action of the medullary center may be inhibited by the
cerebrum (emotions).
Some have located a center in the cerebral cortex (taste cen-
ter), to which it is assumed impulses first travel from the
tongue and which then rouses the proper secreting centers in
the medulla into activity. It seems more likely that the corti-
cal center, if there be one, completes the physiological processes
by which taste sensations are elaborated.
From the influence of drugs (atropin and its antagonist
pilocarpin) it is plain that the gland can be effected through
the blood, though whether wholly by direct action on the cen-
ter, on any local nervous mechanism or directly, on the cells, is
as yet undetermined. It is found that pilocarpin can act long
after section of the nerves. This does not, however, prove that
in the intact animal such is the usual modus operandi of this
or other drugs, any more than the so-called paralytic secretion
after the section of nerves proves that the latter are not con-
cerned in secretion.
We look upon paralytic secretion as the work of the cells
DIGESTION OP FOOD. 315
when gone wrong — passed from under the dominion of the
nerve-centers. Secretion is a part of the natural life-processes
of gland-cells — we may say a series in the long chain of pro-
cesses which are indispensable for the health of these cells.
They must be either secreting cells, or have no place in the nat-
ural order of things. It is to be especially noted that the secre-
tion of saliva continues when the pressure in the ducts of the
gland is greater than that of the blood in its vessels or even
of the carotid ; so that it seems possible that over-importance
has been attached to blood-pressure in secretory processes gen-
erally.
It may, then, be safely assumed that formation of saliva re-
sults in consequence of the natural activity of certain cells, the
processes of which are correlated and harmonized by the nerv-
ous system ; their activity being accompanied by an abundant
supply of blood. The actual outpouring of saliva depends usu-
ally on the establishment of a nervous reflex arc. The other
glands have been less carefully studied, but the parotid is
known to have a double nervous supply from the cerebro-
spinal and the sympathetic systems.
It would appear that, as the vaso-motor changes run paral-
lel with the secretory ones, the vaso-motor and the proper
secretory centers act in concert, as we have, seen holds of the
former and the respiratory center. But it is to our own mind
very doubtful whether the doctrine of so sharp a demarkation
of independent centers, prominently recognized in the physi-
ology of the day, will be that ultimately accepted.
Secretion by the Stomach.— The mucous membrane of St.
Martin's stomach was observed (through an accidental fistulous
opening) to be pale in the intervals of digestion, but flushed
when secreting, which resembled sweating, so far as the flow
of the fluid is concerned. When the man was irritated, the
gastric membrane became pale, and secretion was lessened or
arrested, and it is a common experience that emotions may
help, hinder, or even render aberrant the digestive processes.
While the evidence is thus clear that gastric secretion is
regulated by the nervous system, the way in which this is ac-
complished is very obscure. We know little of either the cen-
ters or nerves concerned, and what we do know helps but
doubtfully to an understanding of the matter, if, indeed, it does
not actually confuse and puzzle.
Digestion can proceed in a fashion after section of the nerves
316
COMPARATIVE PHYSIOLOGY.
going to the stomach, though this has little force as an argu-
ment against nerve influence. We may conclude the subject
by stating that, while the influence of the nervous system over
gastric secretion is undoubted as a fact, the method is not un-
derstood ; and the same remark applies to the secreting activity
of the liver and pancreas.
The Secretion of Bile and Pancreatic Juice. — When the con-
tents of the stomach have reached the orifice of the discharging
bile-duct, a large flow of the biliary secretion takes place, prob-
ably as the result of the emptying of the gall-bladder by the
contraction of its walls and those of its ducts. This is probably
a reflex act, and the augmented flow of bile when digestion is
proceeding is also to be traced chiefly to nervous influences
reaching the gland, though by what nerves or under the gov-
ernment of what part of the nervous centers is unknown.
Very similar statements apply to the secretion of the pancre-
Fig. 257.— Diagram to show influence of food in secretion of pancreatic juice (after
N. O. Bernstein). The abscissa? represent hours after taking food ; ordinates
amount in cubic centigrammes of secretion in ten minutes. Food was taken at
• Ji und C. This diagram very nearly also represents the secretion of bile.
atic glands, though this is not constant, as in the case of bile —
at all events in most animals.
It is known that after food has been taken there is a sudden
DIGESTION OP POOD.
317
increase in the quantity of bile secreted, followed by a sudden
diminution, then a more gradual rise, with a subsequent fall.
Almost the same holds for the pancreas.
It seems impossible to explain tbese facts, especially the
first rapid discharge of fluid apart from the direct influence of
the nervous system.
Upon the whole, the evidence seems to show that the press-
ure in the bile-ducts is greater than in the veins that unite to
make up the portal system; but there are difficulties in the
investigation of such and kindred subjects as regards the liver,
owing to its peculiar vascular supply. It will be borne in miud
that the liver in mammals consists of a mass of blood-vessels,
between the meshes of which are packed innumerable cells, and
that around the latter meander the bile capillaries; that the
portal vein breaks up into the intralobular, from which capil-
laries arise, that terminate in the central interlobular veins,
which make up the hepatic veinlets or terminate in these vessels.
But the structure is complicated by the branches of the hepatic
artery, which, as arterioles and capillaries, enters to some extent
into the formation of the lobular vessels.
A question of interest, though difficult to answer, is the
extent to which the various constituents of bile are manufact-
ured in the liver. Taurin, for example, is present in some of
- wo \-. rT- "Mil "-TT *f .-• '".
Pig. 258.— Lobules of liver, interlobular vessels, and intralobular veins (Sappey). 1, 1,
1, 1, 3, 4, lobules; 2,2, 2, 2, intralobular veins injected with white; 5, 5, 5, 5,5, in-
tralobular vessels tilled with a dark injection.
318
COMPARATIVE PHYSIOLOGY.
the tissues, but whether this is used in the manufacture of
taurocholic acid or whether the latter is made entirely anew,
and possibly by a method in which taurin never appears as
such, is an open question. It is highly probable that a portion
of the bile poured into the intestine is absorbed either as such
or after partial decomposition, the products to be used in
some way in the econo-
§||| ';vA my and presumably in
the construction of bile
by the liver. There are
many facts, including
some pathological phe-
nomena, that point
clearly to the formation
of the pigments of bile
from haemoglobin in
some of its stages of de-
generation.
Pathological.— When
the liver fails to act,
either from derange-
ment of its cells prima-
rily or owing to obstruc-
tion to the outflow of
bile leading to reabsorp-
tion by the liver, bile acids and bile pigments appear in the
urine or may stain the tissues, indicating their presence in ex-
cess in the blood.
This action of one gland (kidneys) for another is highly
suggestive, and especially important to bear in mind in medical
practice, both in treatment and prognosis. The chances of re-
covery when only one excreting gland is diseased are much
greater evidently than when several are involved. Such facts
as we have cited show, moreover, that there are certain common
fundamental principles underlying secretion everywhere— a
statement which will be soon mbre fully illustrated.
Fig. 259. —Portion of transverse section of hepatic
lobule of rabbit magnified 400 diameters (K51H-
ker). b,\b, b, capillary blood-vessels; g, g, g, cap-
■ illary bile-ducts; I, 1,1, liver-cells.
THE NATURE OF THE ACT OF SECRETION.
We are now about to consider some investigations, more
particularly their results, which are of extraordinary interest.
The secreting cells of the salivary, the pancreatic glands,
DIGESTION OF FOOD.
310
and the stomach have been studied by a combination of histo-
logical and, more strictly, physiological methods, to which we
shall now refer. Specimens of these glands, both before and
after prolonged secretion, under stimulation of these nerves,
Fig. 260.— Portion of pancreas of rabbit (after Kiihne and Lea),
at rest; B, during secretion.
A represents gland
were hardened, stained, and sections prepared. As was to be
expected, the results were not entirely satisfactory under these
methods ; however, the pancreas of a living rabbit has been
viewed with the microscope in its natural condition ; and by
this plan, especially when supplemented by the more involved
and artificial method first referred to, results have been reached
which may be ranked among the greatest triumphs of modern
physiology.
Some of these we now proceed to state briefly. To begin
with the pancreas, it has been shown that, when the gland is
not secreting — i. e., not discharging its prepared fluid — or dur-
ing the so-called resting stage, the appearances are strikingly
different from what they are during activity. The cell pre-
sents during rest an inner granular zone and an outer clearer
zone, which stains more readily, and is relatively small in size.
The lumen of the alveolus is almost obliterated, and the in-
dividual cells very indistinct. After a period of secreting
activity, the lumen is easily perceived, the granules have dis-
appeared in great part, the cells as a whole are smaller, and
have a clear appearance throughout. Coincident with the
changes in the gland's cells it is to be noticed that more blood
passes through it, owing to dilatation of the arterioles.
Again, the course of the changes in the salivary glands,
whether of the mucous or serous variety, is very similar. In
320
COMPARATIVE PHYSIOLOGY.
the mucous gland in the resting stage the cells are large, and
hold much clear matter in the interspaces of the cell network ;
Pig. 261. — Section of mucous gland (after Lavdowsky). In A, gland at rest; in B,
after secreting for some time.
and, as this does not stain readily, it can not be ordinary
protoplasm. This, when the gland is stimulated through its
nerves, disappears, leaving the containing cells smaller. It
has become mucin, and may itself be called mucinogen.
It is to be noted that, as the cells become more protoplasmic,
less burdened with the products of their activity, the nucleus
becomes more prominent, suggestive of its having a probable
directive influence over these manufacturing processes.
Substantially the same chain of events has been established
for the serous salivary glands and the stomach, so that we
may safely generalize upon these well-established facts.
It seems clear that a series of changes constructive and, from
one point of view, destructive, following the former are con-
Fig. 202.— ( 'hanges in parotid (serous) gland during secretion (after Langley). A, dur-
ing rest; B, after moderate, C, after prolonged stimulation. Figures partly dia-
grammatic.
stantly going on in the glands of the digestive organs. Proto-
plasm under nerve influence constructs a certain substance,
DIGESTION OF FOOD. 321
which is an antecedent of the final product, which we term a
ferment. It is now customary to speak of these changes as
constructive (anabolic) and destructive (katabolic), though we
have already pointed out (page 258) that this view is, at best,
only one way of looking at the matter, and we doubt if it may
not be cramping and misleading.
We must also urge caution in regard to the conception to
be associated with the use of the terms " resting " and " active "
stage. It is not to be forgotten that strictly in living cells
there is no absolute rest — such means death ; but, if these terms
be understood as denoting but degrees of activity, they need
not mislead. It is also more than probable that in certain of
the glands, or in some animals, the processes go on simultane-
ously ; the protoplasm being renewed, the zymogen, or mother-
ferment, being formed, and the latter converted into actual fer-
ment, all at the same time.
The nature of secretion is now tolerably clear as a wbole ;
though it is to be remembered that this account is but general,
and that there are many minor differences for each gland and
variations that can scarcely be denominated minor for different
animals. Evidently no theory of filtration, no process depend-
ing solely on blood-pressure, will apply here. And if in this,
the best-studied case, mechanical theories of vital processes
utterly fail, why attempt to fasten them upon other glands, as
the kidneys and the lungs, or, indeed, apply such crude concep-
tions to the subtle processes of living protoplasm anywhere or
in any form \
It is somewhat remarkable that an extract of a perfectly
fresh pancreas is not proteolytic ; yet the gland yields such an
extract when it has stood some hours or been treated with a
weak acid. These facts, together with the microscopic appear-
ances, suggested that there is formed a forerunner to the actual
ferment — a zymogen, or mother-ferment, which at the moment
of discharge of the completed secretion is converted into the
actual ferment. We might, therefore, speak of a pepsinogen,
trypsinogen, etc., and, though there may be a cessation in the
series of processes, and no doubt there is in some animals, this
may not be the case in all, or in all glands.
Secretion by the Stomach.— The glands of the stomach differ
in most animals in the cardiac and pyloric regions. In those
of the former zone, both central (columnar) and parietal (ovoid)
cells are to be recognized. It was thought that possibly the lat-
21
#s Mm
-ft1.*)
.vt;«l
5;.
"1
Fig. 263.
Pig. 364.
Fio. 265.
Pig, 268.— Piii- m mucous membrane of stomach in which arc openings of tubular
glands, 1 x 20 (Sappey).
Pig. 364.- Glands of stomach with both central and parietal (ovoid) cells (Heidenhain).
Fi<;. 865.- - Pyloric, glands (Ebstein).
DIGESTION OF FOOD. 323
ter were concerned in the secretion of the acid of the stomach,
hut this is by no means certain. Possibly these, like the demi-
lune cells of the pancreas, may be the progenitors of the central
(chief) cells. The latter certainly secrete pepsin, and probably
also rennet. Mucus is secreted by the cells lining the neck of
glands and covering the mucous membrane intervening be-
tween their mouths. The production of hydrochloric acid by
any act of secretion is not believed in by all writers, some hold-
ing that it is derived from decomposition of sodium chloride,
possibly by lactic acid. So simple an origin is not probable, not
being in keeping with what we know of chemical processes
within the animal body.
Self-Digestion of the Digestive Organs.— It has been found,
both in man and other mammals, that when death follows in a
healthy subject while gastric digestion is in active progress
and the body is kept warm, a part of the stomach itself and
often adjacent organs are digested, and the question is con-
stantly being raised, Why does not the stomach digest itself
during life ? To this it has been answered that the gastric
juice is constantly being neutralized by the alkaline blood ;
and, again, that the very vitality of a tissue gives it the neces-
sary resisting powers, a view contradicted by an experiment
which is conclusive. If the legs of a living frog be allowed
to hang against the inner walls of the stomach of a mammal
when gastric digestion is going on, they will be digested.
The first view (the alkalinity of the blood) would not suffice
to explain why the pancreas, the secretion of which acts best in
an alkaline medium, should not be digested.
It seems to us there is a good deal of misconception about
the facts of the case. Observation on St. Martin shows that
the secretion of gastric juice runs parallel with the need of it,
as dependent on the introduction of food, its quantity, quality,
etc. Now, there can be little doubt that, if the stomach were
abundantly bathed when empty with a large quantity of its
own acid secretion, it would suffer to some extent at least. But
this is never the case ; the juice is carried off and mixed with
the food. This food is in constant motion and doubtless the
inner portions of the cells, which may be regarded as the dis-
charging region (the outer, next tbe blood capillaries, being
the chief manufacturing region of the digestive ferment), are
frequently renewed.
Such considerations, though they seem to have been some-
324
COMPARATIVE PHYSIOLOGY.
what left out of the case, do not go to the bottom of the matter.
Amoeba and kindred organisms do not digest themselves.
Some believe that the little pulsatile vacuoles of the Infusorians
are a sort of temporary digestive cavities.
But, to one who sees in the light of evolution, it must be
clear that a structure could not have been evolved that would
be self -destructive.
The difficulty here is that which lies at the very basis of all
life. We might ask, Why do living things live, since they are
constantly threatened with destruction from within as from
without ? Why do not the liver, kidney, and other glands that
secrete noxious substances, poison themselves ? We can not
in detail explain these things ; but we wish to make it clear
that the difficulty as regards the stomach is not peculiar to that
gland, and that even from the ordinary point of view it has
been exaggerated.
Comparative. — More careful examination of the stomachs of
some mammals has revealed the fact that in several animals,
in which the stomach appears to
be simple, it is in reality com-
pound. There are different
grades, however, which may be
regarded as transition forms be-
tween the true simple stomach
and that highly compound form
of the organ met with in the
ruminants.
It has been shown recently
that the stomach of the hog has
an oesophageal dilatation ; and
that the entire organ may be
divided into several zones with
different kinds of glandular epi-
thelium, etc. These portions
differ in digestive power, in the characteristics of the fluid se-
creted, and other details beyond those which a superficial exam-
ination of this organ would lead one to suspect.
The stomach of the horse represents a more advanced form
of compound stomach than that of the hog, which is not evi-
dent, however, until its glandular structure is examined closely.
The entire left portion of the stomach represents an oesophageal
dilatation lined with an epithelium that closely resembles that
Fig. 266.— Interior of horse's stomach
(after Chauveau). A. left sac; B,
right sac; 0, duodenal dilatation.
DIGESTION OF FOOD.
325
of the oesophagus, and with little if auy digestive function. It
thus appears that the stomach of the horse is in reality smaller,
as a true digestive gland, than it seems, so that a great part of
the work of digestion must he done in the intestine ; though in
this animal, if the food be retained as long as it is in the hog.
which is not, however, the general opinion as regards the
stomach of the horse, salivary digestion may continue for a
considerable period after the food has left the mouth. The
secretion of mucus by the stomach in herbivora is abundant.
As has been already explained, the stomach of ruminants
consists of several compartments which are supplementary to
one another, though genuine gastric digestion does not take
place except in the fourth stomach.
The first and second stomachs being destitute of other than
mucous glands, and lined with a horny epithelium, are to be con-
sidered rather as dilatations of the oesophagus. They answer
admirably the purpose of storehouses for the bulky food in
which the softening process preparatory to mastication goes on.
Pig. 267. — Stomach of the ox scon on its right upper face, tiie aboniasum being de-
pressed (Chauveau). A. rumen, left hemisphere; B, rumen, right hemisphere; ('.
termination of the oesophagus; D, reticulum; E. omasum; P, abomasnm.
326
COMPARATIVE PHYSIOLOGY.
Fig. 268. — Interior of stomach in ruminants; the upper plane of the rumen and reticu-
lum, with the oesophageal furrow (Chauveau). A, left sac of the rumen; B, ante-
rior extremity of that sac turned back on right sac; 0, its posterior extremity, or
left conical cyst; G, section of anterior pillar of rumen; g, g, its two superior
branches; H, posterior pillar of same; h, h, h, its three inferior branches; I, cells
of reticulum; J. oesophageal furrow; K, oesophagus; L, abomasum.
Fig. 269. Stomach of llama (Colin). A, lower extremity of gullet; B, single pillar ot
oesophageal canal: C, superior opening of the psalter; D, reticulum; E, right or
anterior water-cells; W, inferior water-cells; G, fleshy column separating the two
cell groups.
DIGESTION OF FOOD.
327
The reticulum, so called from the peculiar arrangement of
the mucus membrane, is usually regarded as a receptacle for
water more especially ; however, this stomach is to be regarded
both anatomically and lihysiologically as a subdivision of the
first, or at all events as equivalent to that.
The quantity of food that it can hold in the ox is enormous,
(150 to 200 pounds), a condition of things advantageous in an
Fig. 270. — Omasum and abomasum of ox cut open (Smith). A. psalterium, with open-
ing between it and the reticulum at B; P. foldings (plicse) of mucous membrane
at C. fourth stomach.
animal feeding upon substances so poor in nutritive material in
proportion to their bulk and requiring so much mastication to
fit them to be acted on by the digestive juices. The reaction
of tbe first two stomachs is alkaline.
In the camel tribe, water cells are arranged in parallel order
in the rumen. The edges of these are provided with muscular
328
COMPARATIVE PHYSIOLOGY.
fibers constituting sphincters by which their openings inward
may be closed. These cells number several hundred, and are
capable of containing some quarts of water.
Pig. 271.— A. Stomach. of sheep. B. Stomach of musk-deer, m, oesophagus; Rn, ru-
men; Ret. reticulum; Ps, psaltesium; A, Ab, abomasum; Bit, duodenum; Pij,
pylorus (Huxley).
The manyplies is so named from the arrangement of its
mucus membrane in folds, a condition, however, not equally
well marked in all ruminants.
A structure known as the oesophageal canal, (furrow, groove)
communicates with the first three stomachs. During swallow-
ing, its lower portion is raised above the level of the third
stomach, so that it is likely that this is a barrier against the
entrance of all except liquids or soft foods into the manyplies.
It is difficult to make any positive statement as to what other
part it may take in determining the direction of food when en-
tering or leaving the various stomachs. It does not seem to be
C'ssential to return of the cud.
DIGESTION OF FOOD.
329
The abomasum or rennet resembles other forms of true
digestive stomachs in all essential particulars.
While the opening between the first and second stomachs is
large enough to allow of free intercommunication, the reverse
applies to the entrance into the third stomach.
The rumen is nearly always tolerably well filled with food, a
condition of things favorable to its return for remastication.
Pig. 272. — Stomach of horse (after Chauveau). A, cardiac extremity of oesophagus;
B, pyloric ring.
We may conclude that only food in a proper form for the
action of the fourth stomach passes to any extent beyond the
first two.
After the food has been duly softened and has undergone
some fermentative changes in the rumen, leading to the evolution
of gases (C02, H2S) and certain organic acids (acetic, butyric),
it is regurgitated by a process that closely resembles vomiting.
In this the diaphragm and the abdominal muscles, as well
330
COMPARATIVE PHYSIOLOGY.
as the stomach itself and the gullet, take part. Probably as a
result of the descent of the diaphragm and consequent diminu-
tion of the intrathoracic pressure, the asceut of the cud is as-
sisted by an aspiratory process. The returning food is pre-
vented from passing into the nasal chambers by co-ordinated
movements analogous to those of swallowing. The whole pro-
cess is reflex in the same sense as is deglutition.
Normally the rumen always contains considerable liquid, a
portion of which passes up with the cud, but is in great part
returned at once. A ruminant given dry food without water
can not return the cud.
In the second mastication the process is in most ruminants
unilateral ; and as hundreds of cuds are to be chewed, a con-
siderable proportion of the whole day is occupied with rumina-
tion. When a single cud is sufficiently masticated it is swallowed,
Pig. 273.— Stomach of dog (after Chaveau). A, oesophagus; B, pylorus.
and being finely comminuted passes at once through the small
opening between the reticulum and manyplies into the third
stomach, and thence into the abomasum, though possibly on
the way a little may pass into the first two stomachs.
Pathological. — While moderate fullness of the paunch is
DIGESTION OF FOOD. 331
favorable to rumination, extreme distention tends to paralysis
of the muscular coat of the organ, allowing- of the accumulation
of the gases of fermentation which may lead, if not artificially
relieved, to rupture of the organ.
THE ALIMENTARY CANAL OF THE VERTEBRATE.
Amid all variations in this great group, the alimentary canal
has common features, both of structure and function. Through-
out the entire tract muscle cells of the unstriped (involuntary)
kind, arranged in two layers, constitute the motor mechanism
for the transportation of food from one part to another. Out-
side of these is the serous coat, consisting of fibrous and elastic
tissue, and admirably adapted to preserve organs from undue
distention, at the same time providing a smooth external cover-
ing which lessens the friction of one organ against another in
the abdominal cavity ; while folds of such tissue constitute the
omentum for supporting the various organs.
Between the muscular and mucous coats of the organs that
constitute the alimentary canal there is a submucous coat of
loose connective tissue in which ramify blood-vessels, nerves, etc.
It is the mucous coat, however, that is of paramount impor-
tance, and for which all other parts may in some sense be con-
sidered to exist ; for it is from the glands with which it is sup-
plied that the digestive juices are derived, as well as that mucus
which keeps the tract moist and its delicate structures shielded
under all circumstances. The amount of surface provided by
the mucous membrane is increased by its various foldings
(rugce, valvidce conniventes, etc.), so generally present, and
which also allow of distention ; and if the secreting glands
are regarded as minute induplications of this coat, it will
be evident that its total area is much greater than at first ap-
pears.
While each part has glands with structure peculiar to them-
selves, it may be noticed that all the essential epithelium has a
tendency to assume a somewhat cubical form.
The secreting glands of the stomach and intestines are tubu-
lar ; while the salivary glands, the pancreas, and the liver are
masses of cells so packed together as to form great colonies of
cells with lesser subdivisions (lobules), the whole being bound
together by some form of connective tissue, and well supplied
with blood-vessels and nerves, thus constituting organs with a
332 COMPARATIVE PHYSIOLOGY.
covering (capsule) in structure allied to the serous covering of
the stomach and intestines.
Details will be referred to in various parts of the sections
devoted to this subject as far as may be necessary to render
function clear, but we think these few generalizations may tend
to widen the student's field of view, and at the same time lessen
his labor and render it more effective.
THE MOVEMENTS OF THE DIGESTIVE ORGANS.
As with other parts of the body, so in the alimentary tract,
the slower kind of movement is carried out by plain muscular
fibers ; and the movements, as a whole, belong to the class
known as peristaltic ; in fact, it is only at the beginning of the
digestive tract that voluntary (striped) muscle is to be found
and to a limited extent in the part next to this— i. e., in the
oesophagus.
Teeth in the highly organized mammal are remarkable in
being to the least degree living structures of any in the entire
animal, thus being in marked contrast to other organs. The
enamel covering their exposed surfaces is the hardest of all the
tissues, and is necessarily of low vitality. We have already
alluded to the difference in the teeth of different animals, and
their relation to customary food and digestive functions. In
fact, it is clear that the teeth and all the parts of the digestive
system are correlated to one another. The compound stomach
of the ruminants, Avith its slow digestion of a bulky mass of
food which must be softened and thoroughly masticated be-
fore the' digestive juices can attack it successfully, harmonizes
with the powerful jaws, strong muscles of mastication, and
grinding teeth ; and all these in marked contrast with the teeth
of a carnivorous animal with its simple but highly effective
stomach. Compare figures in earlier pages.
Mastication in man is of that intermediate character befit-
ting an omnivorous animal. The jaws have a lateral and for-
ward-and-backward movement, as well as a vertical one, though
the latter is predominant. The upper jaw is like a fixed mill-
stone, against which the lower jaw works as a nether millstone.
The elevation of the jaw is effected by the masseter, temporal,
and internal pterygoid muscles ; depressed by the mylohyoid
and geniohyoid, though principally by the digastric. The jaw
is advanced by the external pterygoids; unilateral contraction
DIGESTION OF FOOD. 333
of these muscles also produces lateral movement of the inferior
maxilla, which is retracted by the more horizontal fibers of the
temporal. The movements of mastication are, of course, very
pronounced in ruminants.
The cheeks and tongue likewise take part in preparing the
food for the work of the stomach, nor must the lips be over-
looked even in man. The importance of these parts is well
illustrated, by the imperfect mastication, etc., when there is
paralysis of the muscles of which they are formed. Even when
there is loss of sensation only, the work of the mouth is done
in a clumsy way, showing the importance of common sensation,
as well as the muscular sense.
Nervous Supply. — The muscles of the tongue are governed by
the hypoglossal nerve ; the other muscles of mastication chiefly
by the fifth. The afferent nerves are branches of the fifth and
glossopharyngeal. It is, of course, important that the food
should be rolled about and thoroughly mixed with saliva (in-
salivation).
Deglutition. — The transportation of the food from the mouth
to the stomach involves a series of co-ordinated muscular acts
of a 'complicated character, by which difficulties are overcome
with marvelous success.
It will be remembered that the respiratory and digestive
tracts are both developed from a common simple tube — a fact
which makes the close anatomical relation between these two
physiologically distinct systems intelligible ; but it also involves
difficulties and dangers. It is well known that a small quantity
of food or drink entering the windpipe produces a perfect
storm of excitement in the respiratory system. The food, there-
fore, when it reaches the oesophagus, must be kept, on the one
hand, from entering the nasal, and on the other, the laryngeal
openings. This is accomplished as follows: When the food has
been gathered into a bolus on tbe back of the tongue, the tip of
this organ is pressed against the hard palate, by which the
mass is prevented from passing forward, and, at the same time,
forced back into the pharynx, the soft palate being raised and
the edges of the pillars of the fauces made to approach the
uvula, which fills up the gap remaining, so that the posterior
nares are closed and an inclined plane provided, over which
the morsel glides. The after-result is said to depend on the
size of the bolus. When considerable, the constrictors of the
pharynx seize it and press it on into the gullet ; when the mor-
334
COMPARATIVE PHYSIOLOGY.
sel is small or liquid is swallowed, it is rapidly propelled on-
ward by the tongue, the oesophagus and pharynx being largely
passive at the time, though contracting slowly afterward; at
-■'■•'/ ■,■ ■'■■: - ■*/;* ;A* -a
\\ '7£&: . /O
Fig. 274.— Cavities of mouth and pharynx, etc.. in man (after Sappey). Section, in
median line, of face and superior portion of neck, designed to show the mouth in
its relations to the nasal fossae, pharynx, and larynx: 1, sphenoidal sinuses; 2, in-
ternal orifice of Eustachian tube; 3, palatine arch; 4, velum pendulum palati; 5.
anterior pillar of soft palate; 6, posterior pillar of soft palate; 7, tonsil; 8, lingual
portion of cavity of pharynx; 9, epiglottis; 10, section of hyoid bone; 11, laryn-
geal portion of cavity of pharynx; 12, cavity of larynx.
the same time the larynx as a whole is raised, the epiglottis
pressed down, chiefly by the meeting of the tongue and itself,
while its cushion lies over the rima glottidis, which is closed
or all but closed by the action of the sphincter muscles of the
larynx, so that the food passes over and by this avenue of life,
not only closed but covered by the glottic lid. The latter is
not so essential as might be supposed, for persons in whom it
DIGESTION OF FOOD. 335
was absent have been known to swallow fairly well. The
ascent of the larynx any one may feel for himself ; and the be-
havior of the pharynx and larynx, especially the latter, may
be viewed by the laryngoscope. The grip of the pharyngeal
muscles and the oesophagus may be made clear by attaching a
piece of food (meat) to a string and allowing it to be partially
swallowed.
The upward movement of food under the action of the
constrictors of the pharynx is anticipated by the closure of
the passage by the palato-glossi of the anterior pillors of the
fauces.
The circular muscular fibers of the gullet are probably the
most important in squeezing on the food by a peristaltic move-
ment, passing progressively over the whole tube, though the
longitudinal also take part in swallowing, perhaps, by steady-
ing the organ.
Deglutition can take place in an animal so long as the
medulla oblongata remains intact ; and the center seems to lie
higher than that for respiration, as the latter act is possible
when, from slicing away the medulla, the former is not. An-
encephalous monsters lacking the cerebrum can swallow, suck,
and breathe.
Food placed in the pharynx of animals when unconscious
is swallowed, proving that volition is not essential to the act ;
but our own consciousness declares that the first stage, or the
removal of the food from the mouth to the pharynx, is volun-
tary.
When we seem to swallow voluntarily there is in reality a
stimulus applied to the fauces, in the absence of food and drink,
either by the back of the tongue or by a little saliva.
It thus appears that deglutition is an act in the main reflex,
though initiated by volition. The afferent nerves concerned
are usually the glossopharyngeal, some branches of the fifth,
and of the vagus. The efferent nerves are those of the numer-
ous muscles concerned.
When food has reached the gullet it is, of course, no longer
under the control of the will.
Section of the vagus or stimulation of this nerve modifies
the action of the oesophagus, though it is known that contrac-
tions may be excited in the excised organ ; but no doubt nor-
mally the movements of the gullet arise in response to natural
nerve stimulation.
336
comparative; physiology.
Comparative. — That swallowing is independent of gravity is
evident from the fact that long-necked animals (horse, giraffe)
can and do usually swallow with the head and neck down, so
that the fluid is rolled up an inclined plane. The peristaltic
nature of the contractions of the gullet can also be well seen in
such animals. In the frog the gullet, as well as the mouth, is
lined with ciliated epithelium, so that in a recently killed ani-
mal one may watch a slice of moistened cork disappear from the
mouth, to be found shortly afterward in the stomach. The rate
of the descent is surprising — in fact, the movement is plainly
visible to the unaided eye.
The Movements of the Stomach. — The stomach of mammals,
including man, is provided with three layers of muscular fibers ;
1. External longitudinal, a continuation of those of the oesopha-
gus. 2. Middle circular. 3. Internal oblique. The latter are
the least perfect, viewed as an investing coat. The pyloric end
of the stomach is best supplied with muscles ; where also there
is a thick muscular ring or sphincter, as compared with which
the cardiac sphincter is weak and ill-developed.
Fig. 275.
Pig. 276.
Pig 275 —Muscular fibers of the stomach of horse; external and middle layers (Chau-
veau). A, fillers of external layer enveloping left sac; B, fibers of middle plane
in right sac: C, fibers of oesophagus.
F„; 270 —Deep and muscular layers exposed by removing mucous membrane from an
everted stomach (Chau'veau). A, deep layer of libers enveloping left sac; B, fibers
of middle plane which alone form the muscular layer of right sac; C, fibers of
oesophagus.
The movements of the stomach begin shortly after a meal
has been taken, and, as shown by observations on St. Martin,
continue for hours, not constantly, but periodically. The effect
DIGESTION OF FOOD. 337
of the conjoint action of the different sets of muscular fibers is
to move the food from the cardiac toward the pyloric end of
the stomach, along1 the greater curvature and back by the lesser
curvature, while there is also, probably, a series of in-and-out
currents to and from the center of the food-mass. The quantity
of food is constantly being lessened by the removal of digested
portions, either by the blood-vessels of the organ or by its
passing through the pyloric sphincter. The empty stomach is
quiescent and contracted, its mucous membrane being thrown
into folds.
The movements of the stomach may be regarded as reflex,
the presence of food being an exciting cause, though probably
not the only one ; and so largely automatic is the central mech-
anisms concerned that but a feeble stimulus suffices to arouse
them, especially at the accustomed time.
Of the paths of the impulses, either afferent or efferent,
little is known. Certain effects follow section or stimulation of
the vagi or splanchnics, but these can not be predicted with
certainty, or the exact relation of events indicated.
It is said that the movements of the stomach cease, even
when it is full, during sleep, from which it is argued that gas-
tric movements do normally depend on the influence of the
nervous system. However, the subject is too obscure at present
for further discussion.
Comparative. — Recent investigations on the stomach of the
pig indicate that in this animal the contents of the two ends of
the stomach may long remain but little mingled ; and such is
certainly the case in this organ among ruminants.
Pathological.— Distention of the stomach, either from excess
of food or gas arising from fermentative changes, or by secre-
tion from the blood, may cause, by upward pressure on the
diaphragm, etc., uneasiness from hampered respiration and
irregularity of the heart, possibly, also, in part traceable to the
physical interference with its movements. After gx*eat and
prolonged distention there may be weakened digestion for a
considerable interval. It seems not improbable that this is to
be explained, not alone by the impaired elasticity (vitality) of
the muscular tissue, but also by defective secreting power. It
is not necessary to impress the lesson such facts convey.
The Intestinal Movements.— The circular fibers play a much
more important part than the longitudinal, being, in fact, much
more developed. It is also to be remembered that nerves in
338 COMPARATIVE PHYSIOLOGY.
the form of plexuses (of Auerbach and Meissner) abound in its
walls.
Normally the movement, slowly progressive, with occasional
haltings, is from above downward, stopping at the ileo-caecal
valve ; the movements of the large gut being apparently mostly
independent.
Movements may be excited by external or internal stimula-
tion, and may be regarded as reflex; in which, however, the
tendency for the central cells to discharge themselves is so great
(automatic) that only a feeble stimulus is required, the normal
one being the presence of food.
It is noticeable in a recently killed animal, or in one in the
last stages of asphyxia, that the intestines contract vigorously.
Whether this is due to the action of blood overcharged with
carbonic anhydride and deficient in oxygen on the centers pre-
siding over the movements, on the nerves in the intestinal
walls, or on the muscle-cells directly, is not wholly clear, but it
is probable that all of these may enter into the result. The
vagus nerve, when stimulated, gives rise to movements of the
intestines, while the splanchnic seems to have the reverse effect ;
but the cerebrum itself has an influence over the movements of
the gut, as is plain from the diarrhoea traceable to unusual fear
or anxiety. There is little to add in regard to the movements
of the large intestine. They are, no doubt, of considerable im-
portance in animals in which it is extensive. Normally they
begin at the ileo-caecal valve.
Defecation. — The removal of the waste matter from the ali-
mentary tract is a complicated process, in which both smooth
and striped muscle, the spinal cord, and the brain take part.
Defecation may take place during the unconsciousness of
sleep or of disease, and so be wholly independent of the will ;
but, as we all know, this is not usually the case. Against ac-
cidental discharge of faeces there is a provision in the sphinc-
ter ani, the tone of which is lost when the lower part of the
spinal cord is destroyed. We are conscious of being able, by an
effort of will, to prevent tbe relaxation of the sphincter or to
increase its holding power, though the latter is probably almost
wholly due to the action of extrinsic muscles ; at all events any
one may convince himself that the latter may be made to take
a great part in preventing faecal discharge, though whether the
tone of the sphincter can be increased or not by volition it is
difficult to say.
DIGESTION OF FOOD. 339
What happens during an ordinary act of defecation is about
as follows : After a long inspiration the glottis is closed ; the
diaphragm, which has descended, remains low, affording, with
the obstructed laryngeal outlet, a firm basis of support for the
action of the abdominal muscles, which, bearing on the intes-
tine, forces on their contents, which, before the act has been
called for, have been lodged mostly in the large intestine ; at
the same time the sphincter ani is relaxed and peristaltic move-
ments accompany and in some instances precede the action of
the abdominal muscles. The latter may contract vigorously on
a full gut without success in the absence of the intestinal peri-
stalsis, as too many cases of obstinate constipation bear witness.
Like deglutition, and unlike vomiting, there is usually both
a voluntary and involuntary part to the act.
Though the will, through the cerebrum, can inhibit defeca-
tion, it is likely that it does so through the influence of the
cerebrum on some center in the cord ; for in a dog, the lumbar
cord of which has been divided from the dorsal, the act is, like
micturition, erection of the penis, and others which are under
the control of the will, still possible, though, of course, per-
formed entirely unconsciously.
Vomiting. — If we consult our own consciousness and observe
to the best of our ability, supplementing information thus
gained by observations on others and on the lower animals, it
will become apparent that vomiting implies a series of co-ordi-
nated movements into which volition does not enter either
necessarily or habitually. There is usually a preceding nausea,
with a temporary flow of saliva to excess. The act is initiated
by a deep inspiration, followed by closure of the glottis.
Whether the glottis is closed during or prior to the entrance
of air is a matter of disagreement. At all events, the dia-
phragm descends and remains fixed, the lower ribs being re-
tracted. The abdominal muscles then acting against this sup-
port, force out the contents of the stomach, in which they are
assisted by the essential relaxation of the cardiac sphincter, the
shortening of the oesophagus by its longitudinal fibers, and the
extension and straightening of the neck, together with the open-
ing of the mouth.
As the expulsive effort takes place, it is accompanied by an
expiratory act which tends to keep the egesta out of the larynx
and carry them onward, though it may also contribute to over-
come the resistance of the elevated soft palate, which serves to
340 COMPARATIVE PHYSIOLOGY.
protect the nasal passages. The stomach and oesophagus are
not wholly passive, though the part they take actively in vom-
iting is variable in different animals.
Retching may be very violent and yet ineffectual when the
cardiac sphincter is not fully relaxed. The pyloric outlet is
usually closed, though in severe and long-continued vomiting
bile is often ejected, which must have reached the stomach
through the pylorus.
Comparative. — The ease with which some animals vomit in
comparison with others is extraordinary, as in carnivora like
our dogs and cats ; a matter of importance to an animal accus-
tomed in the wild state to eat entire carcasses of animals — hair,
bones, etc., included.
The readiness with which an animal vomits depends in great
part on the conformation and relations of the parts of its digest-
ive tract.
The stomach of the human being during infantile life is less
pouched than in the adult, which in part explains the ease with
which very young children vomit.
It is well known that the horse vomits rarely and with great
difficulty. This has been attributed by different writers to va-
rious conditions of a structural kind, such as the length of the
gullet ; the manner in which it enters the stomach (centrally) ;
the pressure of a tightly closing sphincter at this point ; the
valve-like foldings of the mucous membrane at the cardiac
opening ; the small size of the stomach and its sheltered posi-
tion, so that the abdominal muscles can not readily act on it ;
the existence of a considerable length of the oesophagus be-
tween the stomach and diaphragm which is against dilatation
of the orifice by the longitudinal fibers of the gullet ; the open
pylorus, permitting of the gastric contents being driven into the
intestines rather than upward.
But in the ox these peculiarities do not exist; in fact, from a
mechanical point of view, the stracture and illation of parts is
favorable, yet this animal seldom vomits, and never with ease.
Why does the horse vomit after rupture of the stomach when
conditions are less favorable from a mechanical point of view ?
There is the greatest difference as to the readiness with which
different human beings vomit ; moreover, persons that vomit
usually with difficulty may do so very perfectly when suffi-
ciently prepared, as by sea-sickness.
These and many other considerations have led us to conclude
DIGESTION OF FOOD. 341
that, while there is a certain amount of force in the various
views stated briefly above, they do not go to the root of the
matter.
Vomiting is a very complex act, implying numerous muscu-
lar and nervous co-oi'dinations. In the natural wild state the
horse can have but rare necessity to vomit (unlike the carnivora),
hence these co-ordinations have not been organized by habit
and use ; they are foreign to the nature of the animal. After
rupture of the stomach in the horse, and in sea-sickness in man,
the nervous system is profoundly affected and the unusual hap-
pens ; in other words, the necessary muscular and nervous
co-ordinations take place. At all events, we are satisfied that it
lies with the nervous system chiefly.
Similarly, the habit of regurgitating the food is intelligible
in the light of evolution. The fact that mammals are descended
from lower forms in which unstriped muscle-cells go to form
organs that have a rhythmically contractile function, renders
it clear why this function may become, as in ruminants, spe-
cialized in certain parts of the digestive tract ; why carnivora
should vomit readily, and why human subjects should learn to
regurgitate food. There is, so to speak, a latent inherited ca-
pacity which may be developed into actual function. Apart
from this it is difficult to understand such cases at all.
The vomiting center is usually located in the medulla, and
is represented as working in concert with the respiratory center.
But when we consider that there is usually an increased flow
of saliva and other phenomena involving additional central
nervous influence, we see reason to believe in co-ordinated
action implying the use of parts of the central nervous system
not so closely connected anatomically as the respiratory and
vomiting centers are assumed to be.
THE REMOVAL OF DIGESTED PRODUCTS FROM THE
ALIMENTARY CANAL.
The glands of the stomach are simply seci-etive, and all ab-
sorption from this organ is either by blood-vessels directly or
by lymphatics ; at least, such is the ordinary view of the subject
— whether it is not too narrow a one remains to be seen.
It is important to remember that the intestinal mucous
membrane is supplied not only with secreting glands but lym-
phatic tissue, m the form of the solitary and agmiiiated glands
3±2
COMPARATIVE PHYSIOLOGY.
(Peyer's patches) and thickly studded with villi, giving the
small gut that velvety appearance appreciable even by the
naked eye.
It will not be forgotten that the capillaries of the digestive
organs terminate in the veins of the portal system, and that the
blood from these parts is conducted through the liver before it
reaches the general circulation.
Main venous trunk
Right auricle
Vena cava
Hepatic vein
Lymph, gland
Portal system
W @0 Blood vessel, tissue cells.
ilimentary tract
Fig. 277.— Diagram intended to illustrate the general relations of blood and lymph to
metabolism (nutrition), and the method by which the portal, lymphatic, and gen-
eral venous systems are related to the alimentary tract.
The lymphatics of these organs form a part of the general
lymphatic system of the body ; but the peculiar way in which
absorption is effected by villi, and the fact that the lymphatics
of the intestine, etc., at one time (fasting) contain ordinary
lymph and at another (after meals) the products of digestion,
imparts to them a physiological character of their own.
Absorption will be the better understood if we treat now of
lymph and chyle and the lymph vascular system, which were
purposely postponed till the present; though its connection
with the vascular system is as close and important as with the
digestive organs.
The lymphatic system, as a whole, more closely resembles
the venous than the arterial vessels. We may speak of lym-
phatic capillaries, which are, in essential points of structure,
like the arterial capillaries; while the larger vessels may be
compared to veins, though thinner, being provided with valves
and having very numerous anastomoses. These lymphatic
DIGESTION OF FOOD,
343
capillaries begin in spaces between tbe tissue-
cells, from wbicb they take up the effete
lymph. It is interesting to note that there
are also perivascular lymphatics, the exist-
ence of which again shows how close is the
relation between the blood vascular and lym-
phatic systems, and as we would suppose, and
as is actually found to be the case, between
the contents of each.
Lymph and Chyle.— If one compares the
mesentery in a kitten when fasting with the
same part in an animal that was killed some
hours after a full meal of milk, it may be seen
that the formerly clear lines indicating the
course of the lymphatics and ending in glands
have in the latter case become whitish (hence
their name, lacteals), owing to the absorp-
tion of the emulsified fat of the milk.
Microscopic examination shows the chyle
to contain (when coagulated) fibrin, many
Fig. 278.— Valves
of lymphatics
(Sappey).
Fig. 279.— Origin of lymphatics (after Landois). I. From central tendon of diaphragm
of rabbit (semi-diagrammatic) ; 6'. lymph-canals communicating by X with lym-
phatic vessel L\ A, origin of lymphatic by union of lymph-canals; E, E. endothe-
lium. II. Perivascular canal.
344
COMPARATIVE PHYSIOLOGY.
leucocytes, a few developing red corpuscles, an abundance of
fat in the form both of very minute oil-globules and particles
smaller still.
Fig. 280. — Epithelium from duodenum of
rabbit, two hours after having been fed
with melted butter (Funke).
Fig. 281.— Villi filled with fat, from
small intestine of an executed crim
inal, one hour after death (Funke).
There are also present fatty acids, soaps small in quantity
as compared with the neutral fats, also a little cholesterin and
lecithin. But chyle varies very widely even in the same animal
at different times. To the above must be added proteids (fibrin,
serum-albumin, and globulin) ; extractives (sugar, urea, leucin) ;
and salts in which sodium
chloride is abundant.
The composition of
lymph is so similar to that
of chyle, and both to blood,
that lymph might, 'though
only roughly, be regarded
as blood without its red cor-
puscles, and chyle as lymph
with much neutral fat in a
very fine state of division.
The Movements of the
Lymph — comparative. — In
some fishes, some birds, and
:. 882.— Chyle taken from the lacteals and amphibians, there are lymph
thoracic duct, of a criminal executed dur-
ing digestion (Funke). shown leucocytes hearts,
and excessively fine granules of fatty In the frog there are two
Fir
DIGESTION OP FOOD. 345
axillary and two sacral lymph hearts. The latter are, espe-
cially, easily seen, and. there is no doubt that they are under
the control of the nervous system.
In the mammals no such special helps for the propulsion of
lymph exist.
There is little doubt that the blood-pressure is always higher
than the lymph-pressure, and when the blood-vessels are dilated
the fluid within the perivascular lymph-channels is likely com-
pressed; muscular exercise must act on the lymph-channels as
on veins, both being provided with valves, though themselves
readily compressible; the inspiratory efforts, especially when
forcible, assist in two ways: by the compressing effect of the
respiratory muscles, and by the aspirating effect of the negative
pressure within the thorax, producing a similar aspirating
effect within the great veins, into which the large lymphatic
trunks empty. The latter are provided at this point with
valves, so that there is no back-flow; and, with the positive
pressure within the large lymphatic trunks (thoracic duct, etc.),
the physical conditions are favorable to the outflow of lymph
or chyle.
Our knowledge of the nature of the passage of the chyle
from the intestines into the blood is now clearer than it was till
recently, though still incomplete.
The exact structure of a villus is to be carefully considered.
If we assume that the muscular cells in its structure have a
rhythmically contractile function, the blind terminal portion
of the lacteal inclosed within the villus must, after being
emptied, act as a suction -pump to some extent; at all events,
the conditions as to pressure would be favorable to inflow of
any material, especially fluid without the lacteal. The great
difficulty hitherto was to understand how the fat found its way
through the villus into the blood, for, that most of it passes in
this direction there is little doubt.
It is now known that leucocytes (amceboids, phagocytes)
migrate from within the villus outward, and may even reach
its surface, that they take up (eat) fat-particles from the epi-
thelium of the villus, and, independently themselves, carry
them inward, reach the central lacteal and break up, thus re-
leasing the fat. How the fat gets into the covering epithelium
is not yet so fully known — possibly by a similar inceptive pro-
cess; nor is it ascertained what constructive or other chemical
processes it may perform; though it is not at all likely that
Pig. 288
DIGESTION OF FOOD.
347
Fig. 283.— Lymphatic system of horse (Chauveau). A, facial and nasal plexus whose
branches pass to subglossal glands; B, C, parotid lymphatic gland, sending ves-
sels to pharyngeal gland; D. E, large trunks passing toward thorax; P. G, H,
glands receiving superficial lymphatics of neck, a portion of those of limbs, and
those of pectoral parietes; I, junction of jugulars; J, axillary veins; K, summit
of anterior vena cava; L, thoracic duct; M, lymphatics of spleen; in', of stomach;
O, of large colon; S, of small colon; R, lacteals of small intestine, all going to
form two trunks, P. Q, which open directly into receptaculum chyli; T, trunk
which receives branches of sub lumbar glands. U, to which vessels of internal iliac
glands, V, the receptacles of lymphatics of abdominal parietes, pass; W, precrural
glands receiving lymphatics of posterior limb, and which arrive independently in
the abdomen; S, superficial inguinal glands into which lymphatics of the mam-
mce, external generative organs, some superficial trunks of posterior limb, etc.,
pass; Z, deep inguinal glands receiving the superficial lymphatics, Z, of posterior
limbs.
the work of the amoeboid cells is confined to the transport of fat
alone, but that other matters are also thus removed inward to
the lacteal.
When a multitude of facts are taken into account, thei^e
Fig. 284.— Perpendicular section through one of Peyer's patches in the lower part of
the ileum of the sheep (Chauveau). a, a, lacteal vessels in villi; b, b, superficial
layer of lacteal vessels; c, c, deep layer of lacteals; d. cl, efferent vessels provided
with valves; /, Peyer's glands; g, circular muscular layer of wall of intestine; /;,
longitudinal layer.
seems little reason to doubt that so important a process as ab-
sorption can not fail to be regulated by the nervous centers.
348
COMPARATIVE PHYSIOLOGY.
There are two points that are very far from being deter-
mined : the one the fate of the products of digestion ; the other
the exact limit to which digestion is carried. How much — e. g.,
of proteid matter — does actually undergo
conversion into peptone; how much is
converted into leucin and tyrosin; or,
again, what proportion of the albuminous
matters are dealt with as such by the in-
testine without conversion into peptone
at all, either as soluble proteid or in the
form of solid particles ?
1. It is generally believed that solu-
ble sugars are absorbed, usually after
conversion into maltose or glucose, by
the capillaries of the stomach and intes-
tine.
2. There is some positive evidence of
the presence of fats, soaps, and sugars in
unusual amount after a meal in the por-
tal vein, which implies removal from the
intestinal contents by the capillaries,
though, so far as experiment goes, the
fat is chiefly in the form of soaps,
Certain experiments have been made
by ligating the pyloric end of the stom-
ach, by introducing a cannula into the
thoracic duct, so as to continually remove its contents, etc.
But we are surprised that serious conclusions should have been
drawn under such circumstances, seeing that the natural condi-
tions are so altered. What we wish to get at in physiology is
the normal function of parts, and not the possible results after
our interference. Under such circumstances the phenomena
may have a suggestive but certainly can not have a conclusive
value.
It is a very striking fact that little peptone (none, according
to some observers) can be detected even in the portal blood.
True it is, the circulation is rapid and constant, and a small
quantity might escape detection, yet a considerable amount be
removed from the intestine in the space of a few hours by the
capillaries alone. Peptone is not found in the contents of the
thoracic duct.
For a considerable period it has been customary to use the
Fig
. 285. — Intestinal villus
(after Leydig). a, a, a,
epithelial covering; b,b,
capillary network; c, c,
longitudinal muscular
fibers; d, lacteal.
DIGESTION OF FOOD.
349
terms osmosis and diffusion in connection with the functions
of the alimentary canal, and especially the intestinal tract,
as if this thin-walled but complicated organ, or rather collec-
Fig. 286. — A. Villi of man. showing blood-vessels and lacteals; B. Villus of sheep
(after Chauveau).
tion of organs, were little more, so far as absorption is con-
cerned, than a moist membrane, leaving the process of the re-
moval of digested food products to be explained almost wholly
on physical principles.
From such views we dissent. We believe they are opposed
to what we know of living tissue everywhere, and are not sup-
ported by the special facts of digestion. When certain foreign
bodies (as purgatives) are introduced into the blood or the ali-
mentaiy canal, that diffusion takes place, according to physical
laws, may indicate the manner in which the intestine can act;
but even admitting that under such circumstances physical
principles actually do explain the whole, which we do not grant,
it would by no means follow that such was the natural behav-
ior of this organ in the discharge of its ordinary functions.
350
COMPARATIVE PHYSIOLOGY.
When we consider that the blood tends to maintain an equi-
librium, it must be evident that the removal of substances from
the alimentary canal, unless there is to be excessive activity of
str
Fig. 287.— A. Section of villus of rat killed during fat absorption (Schafer). ep, epi-
thelium; str, striated border; c, lymph-cells; c', lymph-cells in epithelium; /, cen-
tral lacteal containing disintegrating corpuscles. B. Mucous membrane of frog's
intestine during fat absorption (Schafer). ep, epithelium; str, striated border; C,
lymph-corpuscles; I, lacteal.
the excretory organs and waste of energy both by them and
the digestive tract, must in some degree depend on the demand
for the products of digestion by the tissues. That there is to
some extent a corrective action of the excretory organs always
going on is no doubt true, and that it may in cases of emergency
be great is also true ; but that this is minimized in ways too
complex for us to follow in every detail is equally true. Diges-
tion waits on appetite, and the latter is an expression of the
needs of the tissues. We believe it is literally true that in a
healthy organism the rate and character of digestion and of
the removal of prepared products are largely dependent on the
condition of the tissues of the body.
Why is digestion more perfect in overfed animals after
a short fast ? The whole matter is very complex, but we think
DIGESTION OF FOOD. 351
it is infinitely better to admit ignorance than attempt to ex-
plain by principles that do violence to our fundamental con-
ceptions of life processes. To introduce " ferments " to explain
so many obscure points in physiology, as the conversion of
peptone in the blood, for example, is taking refuge in a way
that does no credit to science.
Without denying that endosmosis, etc., may play a part in
the vital processes we are considering, we believe a truer view
of the whole matter will be ultimately reached. In the mean
time we think it best to express our belief that we are ignorant
of the real nature of absorption in great part ; but we think
that, if the alimentary tract were regarded as doing for the
digested food (chyle, etc.) some such work as certain other
glands do for the blood, we would be on the way to a truer con-
ception of the real nature of the processes.
It would then be possible to understand that proteids, either
in the form of soluble or insoluble substances, including pep-
tone, might be taken in hand and converted by a true vital
process into the constituents of the blood.
If we were to regard the kidney as manufacturing useful
instead of harmful products, the resemblance in behavior would
in many points be parallel. We have seen that physical expla-
nations of the functions of the kidney have failed, and that it
must be regarded even in those parts that eliminate most water
as a genuine secreting mechanism.
We wish to present a somewhat truer conception of the
lymph that is separated from the capillaries and bathes the
tissues.
We would regard its separation as a true secretion, and not
a mere diffusion dependent wholly on blood-pressure. The
mere ligature of a vein does not suffice to cause an excess of
diffusion, but the vaso-motor nerves have been shown to be
concerned. The effusions that result from pathological pro-
cesses do not correspond with the lymph — that is, the nutrient
material — provided by the capillaries for the tissues. These
vessels are more than mere carriers ; they are secretors — in a
sense they are glands. We have seen that in the foetus they
function both as respiratory and nutrient organs in the allan-
tois and yelk-sac, and, in our opinion, they never wholly lose
this function.
The kind of lymph that bathes a tissue, we believe, depends
on its nature and its condition at the time, so that, as we view
352 COMPARATIVE PHYSIOLOGY.
tissue-lymph, it is not a mere effusion with which the tissues,
for which it is provided, have nothing- to do. The differences
may be beyond our chemistry to determine, but to assume that
all lymph poured out is alike is too crude a conception to meet
the facts of the case. Glands, too, it will be remembered, derive
their materials, like all other tissues, not directly from the
blood, but from the lymph. We believe that the cells of the
capillaries, like all others, are influenced by the nervous system,
notwithstanding that nerves have not been traced terminating
in them.
It is to be borne in mind that the lymph, like the blood,
receives tissue waste-products — in fact, it is very important to
realize that the lymph is, in the first instance, a sort of better
blood — an improved, selected material, so far as any tissue is
concerned, which becomes gradually deteriorated.
We have not the space to give all the reasons on which the
opinions expressed above are founded ; but, if the student has
become imbued with the principles that pervade this work thus
far, he will be prepared for the attitude we have taken, and
sympathize with our departures from the mechanical (physical)
physiology.
We think it would be a great gain for physiology if the use
of the term " absorption." as applied to the alimentary tract,
were given up altogether, as it is sure to lead to the substitu-
tion of the gross conceptions of physical processes instead of
the subtle though at present rather indefinite ideas of vital
processes. We prefer ignorance to narrow, artificial, and er-
roneous views.
Pathological.— Under certain circumstances, of which oue is
obstruction to the venous circulation or the lymphatics, fluid
may be poured out or effused into the neighboring tissues or the
serous cavities. This is of very variable composition, but always
contains enough salts and proteids to remind one of the blood.
• Such fluids are often spoken of as "lymph," though the
resemblance to normal tissue-lymph is but of the crudest kind;
and the condition of the vessels when it is secreted, if such a
term is here appropriate, is not to be compared to the natural
separation of the normal lymph— in fact, were this not so, it
would be identical with the latter, which it is not. When such
effusions take place they are in themselves evidence of altered
(and not merely increased) function.
The Faeces.— The fseces may be regarded in at least a three-
DIGESTION OF FOOD. 353
fold aspect. They contain undigested and indigestible rem-
nants, the ferments and certain decomposition products of the
digestive fluids, and true excretory matters.
In carnivorous and omnivorous animals, including man,
the undigested materials are those that have escaped the action
of the secretions — such as starch and fats — together with those
substances that the digestive juices are powerless to attack,
as horny matter, hairs, elastic tissue, etc.
In vegetable feeders a larger proportion of chlorophyl, cel-
lulose, and starch will, of course, be found.
These, naturally, are variable with the individual, the spe-
cies, and the vigor of the digestive organs at the time.
Besides the above, certain products are to be detected in the
faeces plainly traceable to the digestive fluids, and showing
that they have tmdergone chemical decomposition in the ali-
mentary tract, such as cholalic acid, altered coloring-matters
like urobilin, derivable probably from bilirubin; also ckoles-
terin, fatty acids, insoluble soaps (calcium, magnesium), to-
gether with ferments, having the properties of pepsin and
amylopsin. Mucus is also abundant in the faeces.
We know little of the excretory products proper, as they
probably normally exist in small quantity, and it is not impos-
sible that some of the products of the decomposition of the
digestive juices may be reabsorbed and worked over or excreted
by the kidneys, etc.
There is, however, a recognized non-nitrogenous crystalline
body known as excretin, which contains sulphur, salts, and
pigments, and that may rank perhaps as a true excretion of
the intestine.
It is well known that bacteria abound in the alimentary
tract, though their number is dependent on a variety of circum-
stances, including the kind of food and the condition in which
it is eaten. These minute organisms feed, of course, and to get
their food produce chemical decompositions. Skatol and indol
are possibly thus produced, and give the faecal odor to the con-
tents of the intestine. But as yet our ignorance of these
matters is greater than our knowledge — a remark which ap-
plies to the excretory functions of the alimentary tract gen-
erally.
Pathological.— The facts revealed by clinical and pathologi-
cal study leave no doubt in the mind that the intestine at all
events may, when other glands, like the kidney, are at fault,
23
354 COMPARATIVE PHYSIOLOGY.
undertake an unusual share of excretory work, probably even
to the length of discharging urea.
Obscure as the subject is, and long as it may be before we
know exactly what and how matter is thus excreted, we think
that it will greatly advance us toward a true conception of the
vital processes of the mammalian body if we regard the ali-
mentary tract as a collection of organs with both a secreting
and excreting function ; that what we have been terming ab-
sorption is in the main, at least, essentially secretion or an
allied process ; and that the parts of this long train of organs
are mutually dependent and work in concert, so that when one
is lacking in vigor or resting to a greater or less degree, the
others make up for its diminished activity ; and that the whole
must work in harmony with the various excretory organs, as
an excretor itself, and in unison with the general state of the
economy. We are convinced that even as an excretory mech-
anism one part may act (vicariously) for another.
Of course, in disease the condition of the fasces is an indica-
tion of the state of the digestive organs ; thus color, consistence,
the presence of food in lumps, the odor, and many other points
tell a plain story of work left undone, ill-done, or disordered
by influences operating from within or from without the tract.
The intelligent physician acts the part of a qualified inspector,
surveying the output of a great factory, and drawing conclu-
sions in regard to the kind of work which the operatives have
performed.
THE CHANGES PRODUCED IN THE FOOD IN THE
ALIMENTARY CANAL.
We have now considered the method of secretion, the secre-
tions themselves, and the movements of the various parts of
the digestive tract, so that a brief statement of the results of
all this mechanism, as represented by changes in the food, will
be appropriate. We shall assume for the present that the effects
of the digestive juices are substantially the same in the body as
in artificial digestion.
Among mammals food is, in the mouth, comminuted (except
in the case of the carnivora, that bolt it almost whole, and the
ruminants, that simply swallow it to be regurgitated for fresh
and complete mastication), insalivated, and, in most species,
chemically changed, but only in so far as starch is concerned.
DIGESTION OF POOD.
355
Deglutition is the result of the co-ordinated action of niany
muscular mechanisms, and is reflex in nature. The oesophagus
secretes mucus, which lubricates its walls, and aids mechan-
ically in the transport of the food from the mouth to the stom-
ach. In the stomach, by the action of the gastric juice, food
is further broken up, the proteid covering of fat-cells is digested,
and the structure of muscle, etc., disappears. Proteid matters
become peptone, and in some animals fat is split up into free
fatty acid and glycerin ; but the digestion of fat in the stom-
ach is very limited at best and probably does not go on to
emulsification or saponification. The digestion of starch con-
Fig. 288. — Matters taken from pyloric portion of stomach of dog during digestion of
mixed food (after Bernard), a. disintegrated muscular fibers, striae having disap-
peared; b, c, muscular fibers in which stria; have partly disappeared; d, d, d, glob-
ules of fat; e, e, starch: g, molecular granules.
tinues in the stomach until the reaction of the food-mass be-
comes acid. This in the hog may not be far from one to two
hours, and the amylolytic ferment acts with great rapidity even
without the body. The food is moved about to a certain ex-
tent, so as to expose every part freely to the mucous mem-
brane and its secretions. It is likely that the sugar resulting
from the digestion of starch, the peptones, and, to some ex-
tent, the fat formed (if any), is received into the blood from
the stomach.
356 ' COMPARATIVE PHYSIOLOGY.
As the partially digested mass (chyme) is passed on into the
intestine as a result of the action of the alkaline hile, the para-
peptone, pepsin, and bile-salts are deposited. Certain of the
constituents of digestion are thus delayed, a portion of the pep-
sin is probably absorbed, either altered or unaltered, and pep-
sin is thus got rid of, making the way clear, so to speak, for the
action of trypsin. At all events, digestion in one part of the
tract is antagonized by digestion in another, but we must also
add supplemented.
The fat, which had been but little altered, is emulsified by
the joint action of the bile and pancreatic secretion ; a portion
is saponified, which again helps in emulsification, while an addi-
tional part, in form but little changed, is probably dealt with by
the absorbents.
Proteid digestion is continued, and, besides peptones, nitro-
genous crystalline bodies are formed (leucin and tyrosinj, but
under what conditions or to what extent is not known; though
the quantity is likely very variable, both with the species of
animal and the circumstances, such as quantity and quality of
food ; and it is likely also dependent not a little on the rate of
absorption. It seems altogether probable that in those that use
an excess of nitrogenous food more of these bodies are formed,
and thus give an additional work to the excreting organs, in-
cluding the liver. But the absence of albumin from healthy
faeces points to the complete digestion of proteids in the ali-
mentary canal. Plainly the chief work of intestinal digestion
is begun and carried on in the upper part of the tract, where
the ducts of the main glands are to be found.
The contents of the intestine swarm with bacteria, though
these are probably kept under control, to some extent, by the
bile, the functions of which as an antiseptic we have already
considered.
The removal of fats by the villi will be shortly considered.
The other products of digestion probably find their way into
the general circulation by the portal blood, passing through
the liver, which organ modifies some of them in ways to be
examined later.
The valvulce conniventes greatly increase the surface of the
intestine, and retard the movements of the partially digested
mass, both of which are favorable. The peristaltic movements
of the small gut serve the obvious purpose of moving on the
digesting mass, thus making way for fresh additions of chyme
DIGESTION OP FOOD. 357
from the stomach, and carrying on the more elaborated con-
tents to points where they can receive fresh attention, both
digestive and absorptive.
Comparative. — -In man, the carnivora, and some other groups,
it is likely that digestion in the large intestine is slight, the work
being mostly completed — at all events, so far as the action of
the secretions is concerned — before this division of the tract is
reached, though doubtless absorption goes on there also. The
muscular strength of this gut is important in the act of defe-
cation.
But the great size of the large intestine in ruminants — in
the horse, etc. — together with the bulky character of the food
of such animals, points to the existence of possibly extensive
processes of which we are ignorant. It is generally believed
that food remains but a short time in the stomach of the horse,
and that the caecum is a sort of reservoir in which digestive
processes are in progress, and also for water.
Fermentations go on in the intestine, and probably among
ruminants they are numerous and essential, though our actual
knowledge of the subject is very limited.
The gases found in the stomach are atmospheric air (swal-
lowed) and carbon dioxide, derived from the blood. Those of
the intestine are nitrogen, hydrogen, carbonic anhydride, sul-
phuretted hydrogen, and marsh-gas, the quantity varying con-
siderably with the diet. In herbivora the quantity of C02 and
CH4 is large.
Although our knowledge of the actual processes by which
food is digested in the domestic animals is meager, there are
certain considerations to which it may be well to give promi-
nence at this point.
The whole subject becomes clearer and the way is paved for
more exact and comprehensive knowledge if it be borne in
mind that the entire alimentary tract has a common embryo-
logical origin from the splanchnopleure (Fig. 225, etc.), consist-
ing of outer mesoblast and lining hypoblast, the former giving
rise to the muscular and other less essential structures, the lat-
ter to the all-important glandular epithelium. But of all re-
gions the alimentary tract has been modified in relation to
the development and habits of the animal group. It can not
be too well remembered that digestion is highly complex, with
one organ and one process supplementary to another.
If mastication is imperfect, as in the camivora, gastric diges-
358 COMPARATIVE PHYSIOLOGY.
tion is unusually active, as is well seen in the dog ; if the stom-
ach is capacious the intestine is shorter, also exemplified in
this group. The stomach may be small and the small intes-
tines not lengthy, but the large intestine of enormous size, as in
the horse.
When the quantity of starchy matters found in the food of
the animal is large, provision is made for its digestion in sev-
eral parts of the alimentary tract. This is seen in the horse
and other herbivora. Mastication is fairly complete in these
animals, yet a part of the small stomach of the horse is a sort
of oesophageal dilatation (Fig. 266) in which amylolytic diges-
tion goes on by the action of the swallowed saliva and possibly
by a ferment provided in this region of the organ.
The gastric juice of the horse has been proved capable of
digesting starch, possibly because mixed with the swallowed
saliva. The stomach of the pig is large, and both proteid and
starchy digestion exceedingly active. In the intestines the pro-
cesses are of brief duration, but very effective.
Digestion in the upper part of the small intestines is, in
some animals, as the horse, really a continuation of that in the
stomach ; or, at all events, the contents of the duodenum and
jejunum are usually acid in reaction, so that the digestion
peculiar to one region of the tract does not always abruptly
end when food has left that part. The readiness with which
food passes from the stomach into the intestines is very vari-
able in different animals, and even in the same animal under
different circumstances. In the horse the pyloric orifice seems
never to be very tightly closed, though in most of our domestic
animals the reverse is the case ; and with them the quantity of
undigested material, as fat, that passes into the small intestine
depends on the rate of digestion and absorption in the latter.
In the horse, if water, or even hay, be given after oats a por-
tion of the latter is soon carried on into the intestines, so that
the obvious rule for feeding such an animal is to give the water
and hay before the oats, or, at least, the water and no hay im-
mediately after the oats.
Digestion in the large intestine is of great importance in the
monogastric herbivora, as the horse. The caecum is of enor-
mous size — about twice that of the stomach — and has communi-
cation with the colon by a small opening, so that it furnishes a
sort of supplementary reservoir for digestion as well as for
water. As the results of experiments, it has been concluded
DIGESTION OF FOOD. 359
that food is found in the stomach twelve hours after feeding ;
in the caecum after twenty-four hours, with a residue in the
jejunum ; after forty-eight hours, in the ventral colon, with re-
mains in the caecum; after seventy-two hours, in the dorsal
colon ; and after ninety hours in the dorsal colon and rectum.
The caecum appears to digest large quantities of cellulose,
which does not seem to be affected by either the saliva, gastric,
or pancreatic juices. The pi'ocess is ill understood. In her-
bivora the large intestine takes some very important part — in
digestion and absorption — and we would again remind the stu-
dent that the latter term has been used in a very vague if not
unwarrantable sense. It is important for the practitioner to
bear in mind that nutrient enemata can be utilized for the gen-
eral good of the economy when passed into either the large or
small intestine.
During the suckling period digestion in all the various
groups of animals is probably closely analogous. At this time,
in ruminants, the first three divisions of the stomach are but
slightly developed.
Pathological. — In subjects of a highly neurotic temperament
and unstable nervous system it sometimes happens that im-
mense quantities of gas are belched from an empty stomach or
distend the intestines.
It is known that the oxygen swallowed is absorbed into the
blood, and the carbonic anhydride found in the stomach de-
rived from that fluid.
It will thus be seen that the alimentary tract has not lost its
respiratory functions even in man, and that these may in cer-
tain instances be inordinately developed (reversion).
SPECIAL CONSIDERATIONS.
It is a matter well recognized by those of much experience
in breeding and keeping animals with restricted freedom and
under other conditions differing widely from the natural ones
— i. e., those under which the animals exist in a wild state — that
the nature of the food must vary from that which the untamed
ancestors of our domestic animals used. Food may often with
advantage be cooked for the tame and confined animal. The
digestive and the assimilative powers have varied with other
changes in the organism brought about by the new surround-
ings. So much is this the case, that it is necessary to resort to
360 COMPARATIVE PHYSIOLOGY.
common experience and to more exact experiments to ascertain
the best methods of feeding animals for fattening, for work,
or for breeding. Inferences drawn from the feeding habits of
wild animals allied to the tame to be valuable must always,
before being applied to the latter, be subjected to correction by
the results of experience.
It is now well established by experience that animals kept
in confinement must have, in order to escape disease and attain
the best results on the whole, a diet which not only imitates
that of the corresponding wild forms generally, but even in
details, with, it may be, altered proportions or added constitu-
ents, in consequence of the difference in the environment. To
illustrate: poultry can not be kept healthy confined in a shed
without sand, gravel, old mortar, or some similar preparation ;
and for the best results they must have green food also, as
lettuce, cabbage, chopped green clover, grass, etc. They must
not be provided with as much food as if they had the exercise
afforded by running hither and thither over a large field. We
have chosen this case because it is not commonly recognized
that our domesticated birds have been so modified that special
study must be made of the environment in all cases if they are
not to degenerate. The facts in regard to horned cattle, horses,
and dogs are perhaps better known.
Cooking greatly alters the chemical composition, the me-
chanical condition, and, in consequence, the flavor, the digesti-
bility, and the nutritive value of foods. To illustrate: meat in
its raw condition would present mechanical difficulties, the di-
gestive fluids permeating it less completely; an obstacle, how-
ever, of far greater magnitude in the case of most vegetable
foods. By cooking certain chemical compounds are replaced
by others, while some may be wholly removed. As a rule,
boiling is not a good form of preparing meat, because it with-
draws not only salts of importance, but proteids and the ex-
tractives— nitrogenous and other. Beef-tea is valuable chiefly
because of these extractives, though it also contains a little
gelatin, albumin, and fats.
Meat, according to the heat employed, may be so cooked as
to retain the greater part of its juices within it or the reverse.
With a high temperature (65° to 70° C.) the outside in roasting
may bo so quickly hardened as to retain the juices.
In feeding dogs it is both physiological and economical to
give the animal the broth as well as the meat itself.
DIGESTION OF FOOD. 361
It is remarkable in the highest degree that man's appetite,
or the instinctive choice of food, has proved wiser than our
science. It would he impossible even yet to match, by calcula-
tions based on any data we can obtain, a diet for each man equal
upon the whole to what his instincts prompt. With the lower
mammals we can prescribe with greater success. At the same
time chemical and physiological science can lay down general
principles based on actual experience, which may serve to cor-
rect some artificialities acquired by perseverance in habits that
were not based on the true instincts of a sound body and a
healthy mental and moral nature; for the influence of the
latter can not be safely ignored even in such discussions as the
present. These remarks, however, are meant to be suggestive
rather than exhaustive.
We may with advantage inquire into the nature of hunger
and thirst. These, as we know, are safe guides usually in eat-
ing and drinking.
After a long walk on a warm day one feels thirsty, the
mouth is usually dry; at all events, moistening the mouth,
especially the back of it (pharynx), will of itself partially re-
lieve thirst. But if we remain quiet for a little time the thirst
grows less, even if no fluid be taken. The dryness has been
relieved by the natural secretions. If, however, fluid be intro-
duced into the blood either directly or through the alimentary
canal, the thirst is also relieved speedily. The fact that we
know when to stop drinking water shows of itself that thei'e
must be local sensations that guide us, for it is not possible to
believe that the whole of the fluid taken can at once have en-
tered the blood,
Hunger, like thirst, may be mitigated by injections into the
intestines or the blood. It is, therefore, clear that, while in the
case of hunger and thirst there is a local expression of a need,
a peculiar sensation, more pronounced in certain parts (the
fauces in the case of thirst, the stomach in that of hunger),
yet these may be appeased from within through the medium
of the blood, as well as from without by the contact of food or
water, as the case may be.
Up to the present we have assumed that the changes
wrought in the food in the alimentary tract were identical with
those produced by the digestive ferments as obtained by extracts
of the organs naturally producing them. But for many reasons
it seems probable that artificial digestion can not be regarded as
362 COMPARATIVE PHYSIOLOGY.
parallel with the natural processes except in a very general
way. When we take into account the absence of muscular
movements, regulated according to no rigid principles, hut vary-
ing with innumerable circumstances in all probability ; the ab-
sence of the influence of the nervous system determining the
variations in the quantity and composition of the outflow of the
secretions; the changes in the rate of so-called absorption,
which doubtless influences also the act of the secretion of the
juices — by these and a host of other considerations we are led
to hesitate before we commit ourselves too unreservedly to the
belief that the processes of natural digestion can be exactly
imitated in the laboratory.
What is it which enables one animal to digest habitually
what may be almost a poison to another ? How is it that each
one can dispose readily of a food at one time that at another is
quite indigestible ? To reply that in the one case, the digestive
fluids are poured out and in the other not, is to go little below
the surface, for one asks the reason of this, if it be a fact, as it
no doubt is. When we look further into the peculiarities of
digestion, etc., we recognize the influence of race as such, and
in the race and the individual that obtrusive though ill-under-
stood fact — the force of habit — operative here as elsewhere.
And there can be little doubt that the habits of animals, as to
food eaten and digestive peculiarities established, become or-
ganized, fixed, and transmitted to posterity.
It is probably in this way that, in the course of the evolu-
tion of the various groups of animals, they have come to vary
so much in their choice of diet and in their digestive processes,
did we but know them thoroughly as they are; for to assume
that even the digestion of mammals can be summed up in the
simple way now prevalent seems to us too broad an assump-
tion. The field is very wide, and as yet but little explored.
The law of rhythm is illustrated, both in health and disease,
in striking ways in the digestive tract. An animal long accus-
tomed to eat at a certain hour of the day will experience at that
time not only hunger, but other sensations, probably referable
to secretion of a certain quantity of the digestive juices and to
the movements that usually accompany the presence of food in
the alimentary tract. Hence that '* colic " so common in horses
fed at irregular times and unwisely, after excessive work, etc.
It is well known that defecation at periods fixed, even within
a few minutes, has become an established habit Avith hosts of
DIGESTION OP FOOD. 363
people; and the same is to a degree true of dogs, etc., kept in
confinement, that are taught cleanly habits, and encouraged
therein by regular attention to their needs.
This tendency (rhythm) is important in preserving energy
for higher ends, for such is the result of the operation of this
law everywhere.
The law of correlation, or mutual dependence, is well
illustrated in the series of organs composing the alimentary
tract.
The condition of the stomach has its counterpart in the
rest of the tract ; thus, when St. Martin had a disordered
stomach, the epithelium of his tongue showed corresponding
changes.
We have already referred to the fact that one part may do
extra work to make up for the deficiencies in another.
It is confidently asserted of late that, in the case of persons
long vmable to take food by the mouth, nutritive substances
given by enemata find their way up to the duodenum by anti-
peristalsis. Here, then, is an example of an acquired adaptive
arrangement under the stress of circumstances.
It can not be too much impressed on the mind that in the
complicated body of the mammal the work of any one organ
is constantly varying with the changes elsewhere. It is this
mutual dependence and adaptation — an old doctrine too much
left out of sight in modern physiology — which makes the at-
tempt to completely un ravel vital processes well-nigh hopeless;
though each accumulating true observation gives a better in-
sight into this kaleidoscopic mechanism.
We have not attempted to make any statements as to the
quantity of the various secretions discharged. This is large,
doubtless, but much is probably reabsorbed, either altered or
unaltered, and used over again. In the case of fistula?, the con-
ditions are so unnatural that any conclusions as to the normal
quantity from the data they afford must be highly unsatisfac-
tory. Moreover, the quantity must be very variable, accord-
ing to the law we are now considering. It is well known that
dry food provokes a more abundant discharge of saliva, and
this is doubtless but one example of many other relations be-
tween the character of the food and the quantity of secretion
provided.
Evolution. — We have from time to time either distinctly
pointed out or hinted at the evolutionary implications of the
364 COMPARATIVE PHYSIOLOGY.
facts of this department of physiology. The structure of the
digestive organs, plainly indicating a rising scale of complexity
with greater and greater differentiation of function, is, beyond
question, an evidence of evolution.
The law of natural selection and the law of adaptation,
giving rise to new forms, have both operated, we may believe,
from what can be observed going on around us and in our-
selves. The occurrence of transitional forms, as in the epi-
thelium of the digestive tract of the frog, is also in harmony
with the conception of a progressive evolution of structure and
function. But the limits of space will not permit of the enu-
meration of details.
Summary. — A very brief resume of the subject of digestion
will probably suffice.
Food is either organic or inorganic and comprises proteids,
fats, carbohydrates, salts, and water ; and each of these must
enter into the diet of all known animals. They must also be
in a form that is digestible. Digestion is the reduction of food
to such a form that it may be further dealt with by the aliment-
ary tract prior to being introduced into the blood (absorption).
This is effected in different parts of the tract, the various con-
stituents of food being differently modified, according to the
secretions there provided, etc. The digestive juices contain
essentially ferments which act only under definite conditions of
chemical reaction, temperature, etc.
The changes wrought in the food are the following : starches
are converted into sugars, proteids into peptones, and fats into
fatty acids, soaps, and emulsion ; which alterations are effected
by ptyalin and amylopsin, pepsin and trypsin, and bile and pan-
creatic steapsin, respectively.
Outside the mucous membrane containing the glands are
muscular coats, serving to bring about the movements of the
food along the digestive tract and to expel the faeces, the circu-
lar fibers being the more important. These movements and the
processes of secretion and so-called absorption are under the
control of the nervous system.
The preparation of the digestive secretions involves a series
of changes in the epithelial cells concerned, which can be dis-
tinctly traced, and take place in response to nervous stimula-
tion.
These we regard as inseparably bound up with the healthy
life of the cell. To be natural, it must secrete.
DIGESTION OP FOOD. 365
The blood-vessels of the stomach and intestine and the villi
of the latter receive the digested food for further elaboration
(absorption). The undigested remnant of food and the excre-
tions of the intestine make up the faeces, the latter being ex-
pelled by a series of co-ordinated muscular movements essen-
tially reflex in origin.
THE RESPIRATORY SYSTEM.
In the mammal the breathing1 organs are lodged in a closed
cavity, separated by a muscular partition from that in which
the digestive and certain other organs are contained. This
thoracic chamber may be said to be reserved for circulatory
and respiratory organs which, we again point out, are so related
that they really form parts of one system.
The mammal's blood requires so much aeration (ventilation)
that the lungs are very large and the respiratory system has
become greatly specialized. We no longer find the skin or ali-
mentary canal taking any large share in the process; and the
lungs and the mechanisms by which they are made to move the
gases with which the blood and tissues are concerned become
very complicated.
Our studies of muscle physiology should have made clear
the fact that tissue-life implies the constant consumption of
oxygen and discharge of carbonic anhydride, and that the pro-
cesses which give rise to this are going on at a rapid rate ; so
that the demands of the animal for oxygen constantly may be
readily understood if one assumes, what can be shown, though
less readily than in the case of muscle, that all the tissues are
constantly craving, as it were, for this essential oxygen — well
called '"vital air."
Respiration may, then, be regarded from a physical and
chemical point of view, though in this as in other instances we
must be on our guard against regarding physiological processes
as ever purely physical or purely chemical. The respiratory
process in the mammal, unlike the frog, consists of an active
and a (largely) passive phase. The air is not pumped into the
lungs, but sucked in. So great is the complexity of the lungs
in the mammal, that the frog's lung (which may be readily
understood by blowing it up by inserting a small pipe in the
glottic opening of the animal and then ligaturing the distended
TPIE RESPIRATORY SYSTEM.
367
organ) may be compared to a single infundibulum of the mam-
malian lung.
Assuming that the student is somewhat conversant with the
coarse and fine anatomy of the respiratory organs, we call at-
Fi<i. 2S'J.— Lungs, anterior view (Sappey ). 1, upper lobe of left lung; 2, lower lobe; 3.
fissure; 4, notch corresponding to apex of heart; 5. pericardium; (i. upper lobe of
right lung; 7, middle lobe; 8, lower lobe; 9. fissure: 10, fissure; 11, diaphragm;
12. anterior mediastinum; 13. thyroid gland; 14. middle cervical aponeurosis;" 15.
process of attachment of mediastinum to pericardium; 1G, 10, seventh ribs; 17, 17.
transversales muscles; 18. linea alba.
tention to the physiological aspects of some points in their
structure. The lungs represent a membranous expansion of
368
COMPARATIVE PHYSIOLOGY.
great extent, lined with flattened cells and supporting innu-
merable capillary blood-vessels. The air is admitted to the com-
Fig. 290. — Bronchia and lungs, posterior view (Sappey). 1, 1, summit of lungs; 2,2,
base of lungs; 3, trachea; 4, right bronchus; 5, division to upper lobe of lung; 6,
division to Tower lobe; 7, left bronchus; 8, division to upper lobe; 9, division to
lower lobe; 10, left branch of pulmonary artery; 11, right branch; 12, left auricle
of heart; 13, left superior pulmonary vein; 14, left inferior pulmonary vein; 15,
right superior pulmonary vein; 16, right inferior pulmonary vein; 17, inferior vena
cava; 18, left ventricle of heart; 19, right ventricle.
plicated foldings of this membrane by tubes 'which remain,
throughout the greater part of their extent, open, being com-
posed of cartilaginous rings, completed by soft tissues, of which
plain muscle-cells form an important part, serving to main-
tain a tonic resistance against pulmonary and bronchial press-
ure, as well as serving to aid in the act of coughing, etc.,
so important in expelling foreign bodies or preventing their
ingress.
The bronchial tubes are lined with a mucous membrane,
kept moist by the secretions of its glands, and covered with
ciliated epithelium, as are also the nasal passages, which, by
the outward currents they create, favor diffusion of gases and
removal of excess of mucus. The thoracic walls and the lun^s
THE RESPIRATORY SYSTEM. 369
themselves are covered with a tough but thin membrane lined
with flattened cells, which secrete a small quantity of fluid
that serves to maintain the surrounding parts in a moist con-
Fig. 291. — Mold of a terminal bronchus and a group of air-cells moderately distended
by injection, from the human subject (Robin).
dition. thus lessening friction. The importance of this ar-
rangement is well seen when, in consequence of inflammation
of this pleura, it becomes diy, giving rise during each respira-
tory movement to a friction-sound and a painful sensation.
It will not be forgotten that this membrane extends over the
diaphragm, and that, in consequence of the lungs completely
filling all the space (not occupied by other organs) during every
position of the chest-walls, the costal and pulmonary pleural
surfaces are in constant contact. By far the greater part of
the lung-substance consists of elastic tissue, thus adapting the
principal respiratory organs to that amount of distention and
recoil to which they are ceaselessly subjected during the entire
lifetime of the animal.
24
370
COMPARATIVE PHYSIOLOGY.
Fig. 292.— Section of the parenchyma of the human lung, injecterl through the pul-
monary artery (Schulze). a, a, a, c, c, c, walls of the air-cells; b, small arterial
branch.
THE ENTRANCE AND EXIT OF AIR.
Since the lungs fill up so completely the thoracic cavity,
manifestly any change in the size of the latter must lead to
an increase or diminution in the quantity of air they contain.
Since the air within the respiratory organs is being constantly
robbed of its oxygen, and rendered impure by the addition of
carbonic dioxide, the former must be renewed and the latter
expelled ; and, as mere diffusion takes place too slowly to ac-
complish this in the mammal, this process is assisted by the
nervous system setting certain muscles at work to alter the size
of the chest cavity. Because of the ribs being placed oblicpiely,
it follows that their elevation will result in the enlargement of
the thoracic cavity in the antero-posterior diameter ; and, as the
chest, in consequence, gets wider from above downward, also in
the transverse diameter; which is moreover assisted by the ever-
sion of the lower borders of the ribs; and, if the convexity of the
diaphragm were diminished by its contraction and consequent
descent, it would follow that the chest would be increased in
THE RESPIRATORY SYSTEM.
871
the vertical diameter also. All these events, favorable to
the entrance of air, actually take place through agencies we
must now consider. The student is recommended to look into
the insertion, etc., of the muscles concerned, to which we can
only briefly refer. We have made the descriptions and cuts
applicable to man, so that it may be easy for the student to ver-
ify all essential points on his own person. Respiration in our
domestic animals is in the main as in man.
The act of inspiration commences by the fixation of the
uppermost ribs, beginning with the first two, by means of the
Fig. 293. — Diagram illustrating elevation of ribs in inspiration (B6clard). The dark
lines represent the ribs, sternum, and costal cartilages in inspiration.
Fig. 294.— Diagrammatic representation of action of diaphragm in respiration (Her-
mann). Vertical section throngh second rib on right side. The broken and dot-
ted lines show the amount of the descent of the diaphragm in ordinary and in
deep inspiration.
scaleni muscles, this act being followed up by the contraction of
the external intercostals, leading to the elevation of the other
ribs ; at the same time, the arch of the diaphragm descends in
consequence of the contraction of its various muscular bundles.
Under these circumstances, the air from without must rush in,
or a vacuum be formed in the thoracic cavity ; and, since there
372
COMPARATIVE PHYSIOLOGY,
is free access for the air through the glottic opening, the lungs
are of necessity expanded. This ingoing air has had to over-
come the elastic resistance of the lungs, which amounts to about
Fig. 2f 5.— Apparatus to illustrate relations of intra-thoracic and external pressures
(after Beaunis). A glass bell-jar is provided with a light stopper, through which
passes a branching glass tube fitted with a pair of elastic bags representing lungs.
The bottom of the jar is closed by rubber membrane representing diaphragm. A
mercury manometer indicates the difference in pressure within and without the
bell-jar. In left-hand figure it will be seen that these pressures are equal; in right
(inspiration), the external pressure is considerably greater. At one part (6) an
elastic membrane fills a hole in jar, representing an intercostal space.
five millimetres of mercury in man, as ascertained by tying a
manometer in the windpipe of a dead subject, and then opening
the thorax to equalize the inside and outside pressures, when
the lungs at once collapse and
the manometer shows a rise of
the mercury to the extent indi-
cated above. To this we must
add the influence of the tonic
contraction of the bronchial
muscles before referred to,
though this is probably not very
great.
That there are variations of
intrapulmonary pressure may
be ascertained by connecting a
manometer with one nostril —
the other being closed — or with
the windpipe. The mercury
shows a negative pressure with
each inspiratory, and a positive
Fig. 2%.— Dorsal view of four vertebras
and three attached ribs, showing at-
tachment of elevator muscles of ribs
and intercostal* (after Allen Thom-
son). 1, long and short elevators; 2,
external intercostal; 3, internal in-
tercostal.
THE RESPIRATORY SYSTEM.
373
with each expiratory act. This may amount to from 30 to 70
millimetres with strong inspiration, and 60 to 100 in forcible ex-
piration.
When inspiration ceases, the elastic recoil of the rib carti-
lages and the ribs themselves, and of the sternum, the weight
of these parts and that of the attached muscles, etc., assists in
the return of the chest to its original position, entirely inde-
pendently of the action of muscles. Moreover, with the de-
scent of the diaphragm the abdominal viscera have been thrust
down and compressed together with their included gases; when
this muscle relaxes, they naturally exert an upward pressure.
Putting these events together, it is not difficult to understand
why the air should be squeezed out of the lungs, the elasticity
of which latter is. as wre have shown, an important factor in
itself.
The Muscles of Respiration.— The diaphragm may be con-
sidered the most important single respiratory muscle, and can
of itself maintain respiration. The
scaleni are important as fixators
of the ribs ; the levatores costa-
rum and external intercostals, as
normal elevators. The quadra-
tics lumborum assists the dia-
phragm by fixing the last rib.
These, with the serratus posticus
superior, may be regarded as the
principal muscles called into ac-
tion in an ordinary inspiration.
The muscles used in an ordinary
expiratory act are the internal in-
tercostals, the triungularis sterni,
and serratus posticus inferior.
In forced inspiration the lower
ribs are drawn down and re-
tracted, giving support in their
fixed position to the diaphragm.
The scaleni, pectorales, serratus
magnus, latissimus dorsi, and oth-
ers are called into action ; but.
when dyspnoea becomes extreme,
as in one with a fit of asthma, nearly all the muscles of the
body may be called into play, even the muscles of the face.
Fig. 297.— Laryngoscopy views of
the glottis, etc. (after Quail) and
Czermak). I. Larynx in quiet
breathing. II. During a deep in-
spiration. In this case the rinsrs
of the trachea and commence-
ment of bronchi are visible.
Such a condition is persistent iu
many forms of disease in which
respiration is attended with dif-
ficulty.
374
COMPARATIVE PHYSIOLOGY.
which are not normally active at all or but very slightly in nat-
ural breathing.
Facial and laryngeal respiration is best seen in such ani-
mals as the rabbit, and it is this condition which is ap-
proximated in disordered states in man — in fact, when from
any cause inspiration is very labored (asthma, diphtheria,
etc.).
In man and most mammals, unlike the frog, the glottic
opening is never entirely closed during any part of the respira-
tory act, though it undergoes a rhythmical change of size,
Fig. 2i>8.
Fig. 299.
Fig. 298. — Vertical transverse section of fresh-water mussel (Anodon) through heart
(after Huxley). V. ventricle; a, auricles; r, rectum; p, pericardium; i. inner, o,
outer gill; o', vestibule of organ of Bojanus, fi; f. foot; m.tn, mantle lobes.
Fig. 299.— Gill of fish (perch), to illustrate relations of different blood-vessels, etc.,
concerned in respiration (after Bell). A, branchial artery; B, branchial arch
seen in cross-section; V. branchial vein; a, V, branches of artery and vein re-
spectively.
widening during inspiration and narrowing during expiration,
in accordance with the action of the muscles attached to the
arytenoid cartilages, the action of which may be studied in man
by means of the laryngscope.
The abdominal muscles have a powerful rhythmical action
during forced respiration, though whether they function dur-
THE RESPIRATORY SYSTEM,
375
h\g ordinary quiet breathing is undetermined ; if at all, prob-
ably but slightly. Though the removal of the external inter-
costals in the dog and some other animals reveals the fact
that the internal intercostals contract alternately with the dia-
IV V VI VII VIII IX X XL
XJJ f XII)
XIV
Fig. 300. — Diagram of scorpion, most of the appendages having been removed (after
Huxley), a, mouth; b, alimentary tract; c, anus; d, heart; e. pulmonary sac: /,
position of ventral ganglionated cord; q, cerebral ganglia; T, telson. VII — XX,
seventh to twentieth somite. IV, V, VI, basal joints of pedipalpi and two fol-
lowing pairs of limbs.
phragm, it must not be regarded as absolutely certain that such
is their action when their companion muscles are present, for
Nature has more ways than one of accomplishing the same
purpose — a fact that seems often to be forgotten in reasoning
from experiments. This result, however, carries some weight
with it.
Types of Respiration. — There are among mammals two prin-
cipal types of breathing recognizable — the costal (thoracic) and
abdominal — according as the movements of the chest or the
abdomen (diaphragm) are the more pronounced.
Personal Observation.— The student would do well at this
stage to test the statements we have made in regard to the respira-
tory movements on the human subject especially. This he can
very well do in his own person when stripped to the waist be-
fore a mirror. Many of the abnormalities of the forced respira-
ation of disease may be imitated — in fact, this is one of the
departments of physiology in wThich the human aspects may be
376
COMPARATIVE PHYSIOLOGY,
examined into by a species of experiment on one's self that is
as simple as it is valuable.
Comparative. — It is hoped that the various figures accompa-
nied by descriptions, introduced in this and other chapters, will
#'&■
Fig. 303.
Fig. 301.
Fig. 301.— A. Pulmonary sac. B. Respiratory leaflets of Scorpio occitanus (after
Blancharch.
Fig. 302.— Left pulmonary sac. viewed from dorsal aspect, of a spider (after Duges).
Pm. pulmonary lamella?; Stg. stigma, or opening to former.
make the relations of the circulation and respiration in the va-
rious classes of animals, whether terrestrial or aquatic, evident
Fig. 303 -A. B. Tadpoles with external branchiae (after Huxley), n. nasai sacs; a,
eye; o, ear: /.'. '>. branchiae; m, mouth; z. horny jaws; s, suckers; d, opercular
(or gill) fold. C. More advanced frosts larva, y. rudiment of hind-limb; k. s,
single branchial aperture. Owing to figure not having been reversed, this aper-
ture seems to lie on right instead of left side.
without extended treatment of the subject in the text. What
we are desirous of impressing is that throughout the entire
animal kingdom respiration is essentially the same process: that
THE RESPIRATORY SYSTEM.
377
finally it resolves itself into tissue-breathing — the appropriation
of oxygen and the excretion of carbon dioxide. Since the man-
ner in which oxy-
gen is intro
into the lungs and
foul gases expelled
from them in some
reptiles and amphib-
ians is largely dif-
ferent from the
method of respira-
tion in the mam-
mal, we call atten-
tion to this process
in an animal readily
watched — the com-
mon frog. This
creature, by depress-
ing the floor of the
mouth, enlarges his
air-space in this re-
gion and conse-
quently the air free-
ly enters through
the nostrils ; where-
upon the latter are
closed by a sort of
valve, the glottis
opened and the air
forced into the lungs
by the elevation of
the floor of the
mouth. By a series
of flank movements
the elasticity of the
lungs is aided in
expelling the air
through the now
open nostrils. The
inspiration of the
turtle and some oth-
er reptiles is somewhat similar
Fir,. 304.— General view of air-reservoirs of duck, opened
interiorly, also their relations with principal viscera
of trunk (after Sappey). 1, 1, anterior extremity of
cervical reservoirs ; 2, thoracic reservoir; 3, anterior
diaphragmatic reservoir; 4, posterior ditto; 5, abdom-
inal reservoir; a, membrane forming anterior dia-
phragmatic reservoir; b, membrane forming posterior
ditto; c, section of thoracoabdominal diaphragm: d,
subpectoral prolongation of thoracic reservoir; ■.
pericardium; f, liver; tj. gizzard: //. intestines; in,
lieart; n, n, section of great pectoral muscle above its
insertion into the humerus; o. anterior clavicle; p, pos-
terior clavicle of right side cut and turned outward.
In the case of aquatic animals,
378 COMPARATIVE PHYSIOLOGY.
both invertebrate and vertebrate, excepting mammals, the blood
is freely exposed in the gills to oxygen dissolved in the water as
it is to the same gas mixed with nitrogen in terrestrial animals.
In the land-snail, land-crab, etc., we have a sort of intermediate
condition, the gills being kept moist. It is not to be forgotten,
however, that normally the respiratory tract of mammals is
never other than slightly moist.
THE QUANTITY OF AIR RESPIRED.
We distinguish between the quantity of air that usually is
moved by the thorax and that which may be respired under
special effort, which, of course, can never exceed the capacity
of the respiratory organs.
Accordingly, we recognize: 1. Tidal air, or that which
passes in and out of the respiratory passages in ordinary quiet
breathing, amounting to about 500 cc, or thirty cubic inches.
2. Complemental air, which may be voluntarily inhaled by a
forced inspiration in addition to the tidal air, amounting to
1,500 cc, or about 100 cubic inches. 3. Supplemental {reserve)
air, which may be expelled at the end of a normal respiration
— i. e., after the expulsion of the tidal air, and which represents
the quantity usually left in the lungs after a normal quiet ex-
piration, amounting to 1,500 cc. 4. Residual air, which can
not be voluntarily expelled at all, amounting to about 2,000 cc,
or 120 cubic inches. Although these quantities have been esti-
mated for man, probably a similar relation (proportion) between
them holds for the domestic animals.
The vital capacity is estimated by the quantity of air that
may be expired after the most forcible inspiration. This will,
of course, vary with the age, which determines largely the elas-
ticity of the thorax, together with sex, position, height, and a
variety of other circumstances. But, inasmuch as the result
may be greatly modified by practice, like the power to expand
the chest, the vital capacity is not so valuable an indication as
might at first be supposed.
It is important to bear in mind that the tidal air is scarcely
more than sufficient to fill the upper air-passages and larger
bronchi, so that it requires from five to ten respirations to re-
move a quantity of air inspired by an ordinary act. Very much
must, therefore, depend on diffusion, the quantity of air remain-
ing in the lungs after each breath being the sum of the residual
THE RESPIRATORY SYSTEM. 379
and reserve air, or about 3,500 cc. (220 cubic iucbes). Consider-
ing the creeping slowness of the capillary circulation, it would
not be supposed that the respiratory process in its essential
parts should be the rapid one that a greater movement of the
air would imply.
THE RESPIRATORY RHYTHM.
In man, and most of our domestic mammals, a definite
though somewhat different relation between the cardiac and
respiratory movements obtains, there being about three to five
heart-beats to one respiration, which would make the rate of
breathing in man about sixteen to eighteen per minute. Usual-
ly, of course, the largest animals have the slower pulse and res-
piration ; and this is an invariable rule for the varieties of a
species, as observable in the canine race, to mention a well-
known instance. The horse breathes 9 to 12 times in a minute;
the ox 15 to 20 ; the sheep 13 to 17 ; and the dog 15 to 20.
The rate of the respiratory movements is to some extent a
measure of the rapidity of the oxidative processes in the body,
as witness the slow and intermittent breathing of cold-blooded
animals as compared with the more rapid respiration of birds
and mammals (Fig. 305).
Pathological. — Any condition that lessens the amount of re-
spiratory surface, or diminishes the mobility of the chest-walls,
is usually accompanied by accelerated movements, but beneath
this is the demand for oxygen, part of the avenues by which
this gas usually enters having been closed or obstructed by the
disease. So that it is not surprising that, in consequence of
the effusion of fluid into the thoracic cavity, leading to the
compression of the lung, the opposite one should be called into
more frequent use, and even enlarge to meet the demand.
These facts show how urgent is the need for constant ventila-
tion of the blood, and at the same time how great is the power
of adaptation to meet the emergency.
The difference between the inspiratory and the expiratory
rhythm may be gathered by watching the movements of the
bared chest, or more accurately from a graphic record. It is
usually considered that expiration is only slightly longer than
inspiration, and that any marked deviation from this relation
should arouse suspicion of disease. Normally the respiratory
pause is very slight, so that inspiration seems to follow directly
380
COMPARATIVE PHYSIOLOGY.
on expiration ; though the latter act reminds us of the pro-
longation of the ventricular systole after the blood is expelled.
10
Fig 305 —Tracings of respiratory movements of individuals belonging to different
groups Of the animal kingdom (after Thanhoffer). Differences in depth, frequency,
and especially regularity, are very noticeable. 1, fish; 2, tortoise; 3, adder (nv
winter); 4, boa-constrictor (in summer); 5, frog; 0, alligator; 7, lizard; 8, canary-
bird; 0. adult dog; 10, rabbit; 11, man; 12, dog; 13, horse. Compare these, and
note that in nl respiration is shallow, and in ml deep.
THE RESPIRATORY SYSTEM.
381
If, in the tracing, the small waves on the upper part of the
expiratory curve really represent the effect of the heart-beat,
it makes it easier to understand how such might assist in venti-
lating the blood when the respirations occur only once in a
considerable interval and very feebly then, as in hibernating
animals or individuals that have fainted ; though it must be
remembered that diffusion is a ceaseless process in all living
vertebrates.
FrG. 306. — Tracings of respiration of horse when at rest and after exercise (after Than-
hoffer). /, inspiration; E, expiration. Spaces between vertical lines indicate
time periods of one second each. 1, animal standing at rest; 3, after walk of few
minutes; 7 and 8, after trotting; 9, after a brief rest; 11, after trotting and run-
ning for some minutes; 17, after resting from last for a short time; 51, tracing at
end of experiment.
It is scarcely necessary to point out that the respiratory
movements are increased by exercise, emotions, position, sea-
son, hour of the day, taking meals, etc.
Respiratory Sounds.— The entrance and exit of air are ac-
companied by certain sounds, which vary with eacli part of the
382 COMPARATIVE PHYSIOLOGY.
respiratory tract. To these sounds names have been given, but
as they are somewhat inconstant in their application, or at least
have several synonyms, we pass them by, recommending the
student to actually learn the nature of the respiratory murmurs
by listening to the normal chest in both man and the lower ani-
mals. With the use of a double stethoscope he may practice
upon himself, though not so advantageously as in the case of
the heart.
The sounds are caused in part by the friction of the air,
though they are probably complex, several factors entering
into their causation.
COMPARISON OF THE INSPIRED AND EXPIRED AIR.
The changes that take place in the air respired may be brief-
ly stated as follows :
1. Whatever the condition of the inspired air, that expired
is about saturated with aqueous vapor — i. e., it contains all that
it is capable of holding at the existing temperature.
2. The temperature of the expired air is about that of the
blood itself, so that if the air is very cold when breathed, the
body loses a great deal of its heat in warming it. The expired
air of the nasal passages is slightly warmer than that of the
mouth.
3. Experiment shows that the expired air is really dimin-
ished in volume to the extent of from one fortieth to one fif-
tieth of the whole. Since two volumes of carbonic anhydride
require for their composition two volumes of oxygen, if the
amount of the former gas expired be not equal to the amount
of oxygen inspired, some of the latter must have been used to
CO
form other combinations. -=-^, amounting to rather less than
1, is called the respiratory coefficient.
4. The difference between inspired and expired air in man
may be gathered from the following :
Inspired air.
Expired air.
Oxygen.
Nitrogen.
Carbonic dioxide.
20-810
79-150
0-040
10-083
79-587
4-380
From which the most important conclusions to be drawn
are, that the expired air is poorer in oxygen to the extent of 4
to 5 per cent, and richer in carbonic anhydride to somewhat
THE RESPIRATORY SYSTEM. 333
less than this amount. A similar relationship may be considered
to hold for the domestic animals, the quantities varying, of course.
From experiment it has been ascertained that the amount
of carbonic dioxide is for the average man 800 grammes (406
litres, equivalent to 218*1 grammes carbon) daily, the oxygen
actually used for the same period being 700 grammes. But the
variations in such cases are very great, so that these numbers
must not be interpreted too rigidly. Experience proves that,
while chemists often work in laboratories in which the per-
centage of carbonic anhydride (from chemical decompositions)
reaches 5 per cent, an ordinary room in which the amount of
this gas reaches 1 per cent is entirely unfit for occupation. This
is not because of the amount of the carbon dioxide present, but
of other impurities which seem to be excreted in proportion to
the amount of this gas, so that the latter may be taken as a
measure of these poisons.
What these are is as yet almost entirely unknown, but that
they are poisons is beyond doubt. Small effete particles of
once living protoplasm are carried out with the breath, but
these other substances are got rid of from the blood by a vital
process of secretion (excretion), we must believe; which shows
that the lungs to some degree play the part of glands, and that
their whole action is not to be explained as if they were merely
moistened bladders acting in accordance with ordinary physi-
cal laws.
An estimation of the amount of atmospheric air required
may be calculated from data already given.
Thus, assuming that a man gives up at each breath 4 per
cent of carbon dioxide to the 500 cc. of tidal air he expires, and
breathes, say, seventeen times a minute, we get for the amount
of air thus charged in one hour to the extent of 1 per cent :
500 x 4 x 17 x 60 = 2,040,000 cc, or 2,040 litres.
But if the air is to be contaminated to the extent of only TV
per cent of carbonic anhydride, the amount should equal at
least 2,040 x 10 hourly. A very much larger quantity would,
of course, be required for a horse or an ox.
RESPIRATION IN THE BLOOD.
It may be noticed that arterial blood kept in a confined space
grows gi'adually darker in color, and that the original bright-
scarlet hue may be restored by shaking it up with air. When
384 COMPARATIVE PHYSIOLOGY.
the blood has passed through the capillaries and reached the
veins, the color has changed to a sort of purple, characteristic
of venous blood. Putting these two facts together, we are led
to suspect that the change has been caused in some way by
oxygen. Exact experiments with an appropriate form of blood-
pump show that from one hundred volumes of blood, whether
arterial or venous, about sixty volumes of gas may be obtained :
that this gas consists chiefly of oxygen and carbonic anhydride,
but that the proportions of each present depends upon whether
the blood is arterial or venous.
The following table will make this clear :
Arterial blood
Venous blood.
:ygen. Cai
■bonic anhydride.
Nitrogen.
20
40
1-2
8-12
46
1-2
from 100 volumes of blood at 0° C. and 760 millimetre pressure.
Arterial blood, then, contains 8 to 12 per cent more oxygen
and about 6 per cent more carbonic dioxide than venous blood.
It is not, of course, true, as is sometimes supposed, that arterial
blood is " pure blood " in the sense that it contains no carbonic
anhydride, as in reality it always carries a large percentage of
this gas.
The Conditions under which the Gases exist in the Blood —
If a fluid, as water, be exposed to a mixture of gases which it
can absorb under pressure, it is found that the amount taken up
depends on the quantity of the particular gas present independ-
ent of the presence or quantity of the others ; thus, if water be
exposed to a mixture of oxygen and nitrogen, the quantity of
oxygen absorbed will be the same as if no nitrogen were pres-
ent— i. e. , the absorption of a gas varies with the partial press-
ure of that gas in the atmosphere to which it is exposed. But
whether blood, deprived of its gases, be thus exposed to oxygen
under pressure, or whether the attempt be made to remove this
gas from arterial blood, it is found that the above-stated law
does not apply.
When blood is placed under the exhaustion-pump, at first
very little oxygen is given off; then, when the pressure is con-
siderably reduced, the gas is suddenly liberated in large quan-
tity, and after this comparatively little. A precisely analogous
course of events takes place when blood deprived of its oxygen
is submitted to this gas under pressure. On the other hand,
if these experiments be made with serum, absorption follows
THE RESPIRATORY SYSTEM. 385
according to the law of pressures. Evidently, then, if the oxy-
gen is merely dissolved in the hlood, such solution is peculiar,
and we shall presently see that this supposition is neither ne-
cessary nor reasonable.
HEMOGLOBIN AND ITS DERIVATIVES.
Haemoglobin constitutes about -^\ of the corpuscles, and,
though amorphous in the living blood-cells, may be obtained
in crystals, the form of which varies with the animal ; indeed,
in many animals this substance crystallizes spontaneously on
the death of the red cells. It is unique among albuminous
compounds in being the only one found in the animal body
that is susceptible of crystallization. Its estimated composi-
tion is:
Carbon 53'85
Hydrogen 7"32
Nitrogen 16 '17
Oxygen 2184
Iron -43
Sulphur -39
together with 3 to 4 per cent of water of crystallization.
The formula assigned is: CeooHgeoOivgN^FeSs. The molecu-
lar constitution is not known, and the above formula is merely
an approximation, which will, however, serve to convey an idea
of the great complexity of this compound. The presence of
iron seems to be of great importance. If not the essential re-
spiratory constituent, certainly the administration of this metal
in some form proves very valuable when the blood is deficient
in haemoglobin.
This substance can be recognized most certainly by the spec-
troscope. The appearances vary with the strength of the solu-
tion, and, as this test for blood (haemoglobin) is of much prac-
tical importance, it will be necessary to dwell a little upon the
subject; though, after a student has once recognized clearly the
differences of the spectrum appearances, he has a sort of knowl-
edge that no verbal description can convey. This is easily ac-
quired. One only needs a small, flat-sided bottle and a pocket-
spectroscope. Filling the bottle half full of water, and getting
the spectroscope so focused that the Fraunhofer lines appear
distinctly, blood, blood-stained serum, a solution of haemoglo-
bin crystals, or the essential substance in any form of dilute
25
386
COMPARATIVE PHYSIOLOGY.
solution, may be added drop by drop till changes in the spec-
trum in the form of dark bands appear. By gradually increas-
ing the quantity, ap-
pearances like those fig-
ured below may be ob-
served, though, of course,
much will depend on the
thickness of the layer of
fluid as to the quantity
to be added before a par-
ticular band comes into
view.
When wishing to be
precise, we speak of the
most highly oxidized
form of haemoglobin as
oxy-haemoglobin (O-H),
and the reduced form as
haemoglobin simply, or
reduced haemoglobin (H) .
By a comparison of
the spectra it will be seen
that the bands of oxy-
haemoglobin lie between
the D and E lines; that
the left band near D is
always the most definite
in outline and the most
pronounced in every re-
spect except breadth ; that it is in weak solutions the first to ap-
pear, and the last to disappear on reduction ; that there are two
instances in which there may be a single band from haemoglo-
bin— in the one case when the solution is very dilute and when
it is very concentrated. These need never be mistaken for each
other nor for the band of reduced haemoglobin. The latter is a
hazy broad band with comparatively indistinct outlines, and
darkest in the middle.
It will be further noticed that in all these instances, apart
from the bands, the spectrum is otherwise modified at each
end, so that the darker the more centrally placed characteristic
bands, the more is the light at the same time cut off at each
end of the spectrum.
Fig. 307.— Crystallized haemoglobin (Gautier). a, b,
crystals from venous blood of man; c, from
blood of cat; d, of Guinea-pig; e, of marmot;
f, of squirrel.
THE RESPIRATORY SYSTEM.
387
388 COMPARATIVE PHYSIOLOGY.
If, now, to a specimen showing the two bands of oxy-haemo-
globin distinctly a few drops of ammonium sulphide or other
reducing agent be added, a change in the color of the solution
will result, and the single hazy band characteristic of hemo-
globin will appear.
It is not to be supposed, however, that venous blood gives
this spectrum. Even after asphyxia it will be difficult to see
this band, for usually some of the oxy-ha^moglobin remains
reduced ; but it is worthy of note, as showing that the appear-
ances are normal, that the blood, viewed through thin tissues
when actually circulating, whether arterial or venous, gives
the spectrum of oxy-bsemoglobin. At the same time there can
be no doubt that the changes in color which the blood under-
goes in passing through the capillaries is due chiefly to loss of
oxygen, as evidenced by the experiments before referred to ; and
the reason that tbe two bands are always to be seen in venous
blood is simply that enough oxy-haamoglobin remains to give
the two-band spectrum which prevails over that of (reduced)
haemoglobin. We are thus led by many paths to the important
conclusion that the red corpuscles are oxygen-carriers, and,
though this may not be and probably is not their only func-
tion, it is without doubt their principal one. Of their oxygen
they are being constantly relieved by the tissues; hence the
necessity of a circulation of the blood from a respiratory point
of view.
There are other gases that can replace oxygen and form
compounds with haemoglobin ; hence we have CO-hasmoglobin
and NO-haemoglobin, which in turn are replaced by oxygen with
no little difficulty — a fact which explains why carbonic oxide is
so fatal when respired, and, as it is a constituent of illuminat-
ing gas, the cause of the death of those inhaling the latter is
often not far to seek. Blood may, in fact, be saturated with
carbonic oxide by allowing illuminating gas to pass through it,
when a change of color to a cherry red may be observed, and
which will remain in spite of prolonged shaking up with air or
attempts at reduction with the usual reagents. Haemoglobin
may be resolved into a proteid (globin) not well understood,
and hcematin. This happens when the blood is boiled (perhaps
also in certain cases of lightning-stroke), and when strong acids
are added. Haematin is soluble in dilute acids and alkalies, and
has then characteristic spectra. Alkaline haematin may be re-
duced ; and, as the iron can be separated, resulting in a change
THE RESPIRATORY SYSTEM. 389
of color to brownish red, after which there are no longer any
reducing- effects, it would seem that the oxygen-carrying power
and iron are associated. This iron-free haernatin is named
hcematorporphyrin or hcematoin.
Hcemin is hydrochlorate of haernatin (Teichinanms crystals),
and may be formed by adding glacial acetic acid and common
salt to blood, dried blood-clot, etc., and heating to boiling. This
is one of the best tests for blood, valuable in medico-legal and
other cases.
When oxy-haemoglobin stands exposed to the air, or when
diffused in urine, it changes color and becomes, in fact, another
substance — methcemoglobin, irreducible by other gases (CO, etc.),
and not surrendering its oxygen in vacuo, though giving it up
to ammonium sulphide, becoming again oxy-haemoglobin, when
shaken up with atmospheric air. Its spectrum differs from
that of oxy-haemoglobin in that it has a band in the red end of
the spectrum between the C and D lines. Hcematoidin is some-
times found in the body as a remnant of old blood-clots. It is
probably closely allied to if not identical with the bilirubin
of bile.
Comparative. — "While haemoglobin is the respiratory agent in
all the groups of vertebrates, this is not true of the inverte-
brates. Red blood-cells have as yet been found in but a few
species, though haemoglobin does exist in the blood plasma of
several groups, to one of which the earth-worm and several
other annelids belong. It is interesting to note that the respir-
atory compound in certain families of crustaceans, as the com-
mon crab, horseshoe-crab (limulus), etc., is blue, and that in
this substance copper seems to take the place of iron.
The Nitrogen, and the Carbon Dioxide of the Blood. -The
little nitrogen which is found in about equal quantity in venous
and arterial blood seems to be simply dissolved. The relations
of carbonic anhydride are much more complex arid obscure.
The main facts known are that — 1. The quantity of tin's gas is
as great in serum as in blood, or, at all events, the quantity in
serum is very large. 2. The greater part may be extracted by
an exhaustion-pump ; but a small percentage (2 to 5 volumes
per cent) does not yield to this method, but is given off when
an acid is added to the serum. 3. If the entire blood be sub-
jected to a vacuum, the whole of the C02 is given off.
From these facts it has been concluded that the greater part
of the C02, exists in the plasma, associated probably with sodium
390 COMPARATIVE PHYSIOLOGY.
salts, as sodium bicarbonate, but that the corpuscles in some way
determine its relations of association and disassociation. Some
think a good deal of this gas is actually united with the red cor-
puscles.
We may now inquire into the more intimate nature of respi-
ration in the blood. From the facts we have stated it is obvious
that respiration can not be wholly explained by the Henry-Dal-
ton law of pressures or any other physical law. It is also plain
that any explanation which leaves out the principle of pressure
must be incomplete.
While there is in oxy-haemoglobin a certain quantity of
oxygen, which is intra-molecular and incapable of removal by
reduction of pressure, there is also a portion which is subject
to this law, though in a peculiar way ; nor is tbe question of
temperature to be excluded, for experiment shows that less
oxygen is taken up by blood at a high than at a low tempera-
ture.
We have learned that in ordinary respiration, the propor-
tion of carbonic dioxide and oxygen in different parts of the
respiratory tract must vary greatly ; the air of necessity being
much less pure in the alveoli than in the larger bronchi.
It is customary to speak of the oxygen of oxy-hsemoglobin
as being in a state of u loose chemical combination." The en-
tire truth seems to lie in neither view, though both are partially
correct. The view entertained by some physiologists, to the
effect that diffusion explains the whole matter, so far, at least,
as carbonic anhydride is concerned, and that the epithelial cells
of the lung have no share in the respiratory process, does not
seem to be in harmony either with the facts of respiration or
with the laws of biology in general. Why not say at once that
the facts of respiration show that, here as in other parts of the
economy, while physical and chemical laws, as we know them,
stand related to the vital processes, yet, by reason of being vital
processes, we can not explain them according to the theories of
either physics or chemistry ? Surely this very subject shows
that neither chemistry nor physics is at present adequate to
explain such processes. It is, of course, of value to know the
circumstances of tension, temperature, etc., under which respi-
ration takes place. We, however, maintain that these are con-
ditions only — essential no doubt, but though important, that
they do not make up the process of respiration. But, because
we do not know the real explanation, let us not exalt a few
THE RESPIRATORY SYSTEM. ' 39 1
facts or theories of chemistry or physics into a solution of a
complex problem. Besides, some of the experiments on which
the conclusions have been based are questionable, inasmuch as
they seem to induce artificial conditions in the animals oper-
ated upon ; and we have already insisted on the blood being
regarded as a living tissue, behaving differently in the body
and when isolated from it, so that even in so-called blood-gas
experiments there may be sources of fallacy inherent in the
nature of the case.
Foreign Gases and Respiration.— These are divided into:
1. Indifferent gases, as N, H, CH4, which though not in
themselves injurious, are entirely useless to the economy.
2. Poisonous gases, fatal, no matter how abundant the nor-
mal respiratory food may be. They are divisible into : (a) those
that kill by displacing oxygen, as NO, CO, HCN ; (b) narcotic
gases, as C02, N20, producing asphyxia when present in large
quantities; (c) reducing gases, as H2S, (NH4)2S, PH3, AsH3, C2N2,
which rob the haemoglobin of its oxygen.
There are probably a number of poisonous products, some
of them possibly gases, produced by the tissues themselves and
eliminated normally by the respiratory tract ; and these are
doubtless greatly augmented, either in number or quantity, or
both, when other excreting organs are disordered.
RESPIRATION IN THE TISSUES.
We first direct attention to certain striking facts :
1. An isolated (frog's) muscle will continue to contract for
a considerable period and to exhale carbon dioxide in the total
absence of oxygen, as in an atmosphere of hydrogen ; though,
of course, there is a limit to this, and a muscle to which either
no blood flows, or only venous blood, soon shows signs of
fatigue. 2. In a frog, in which physiological saline solution
has been substituted for blood, the metabolism will continue,
carbonic anhydride being exhaled as usual. 3. Substances,
which are readily oxidized, when introduced into the blood of
a living animal or into that blood when withdrawn uudergo
but little oxidative change. 4. An entire frog will respire car-
bonic dioxide for hours in an atmosphere of nitrogen.
Such facts as these seem to teach certain lessons clearly. It
is evident, first of all, that the oxidative processes that give rise
to carbon dioxide occur chiefly in the tissues and not in the
392 COMPARATIVE PHYSIOLOGY.
blood ; that in the case of muscle the oxygen that is used is first
laid by, banked as it were against a time of need, in the form of
infra-molecular oxygen, which is again set free in the form of
carbon dioxide, but by what series of changes we are quite un-
able to say. Though our knowledge of the respiratory processes
of muscle is greater than for any other tissue, there seems to
be no reason to believe that they are essentially different else-
where. The advantages of this banking of oxygen are, of
course, obvious ; were it otherwise, the life of every cell must
be at the mercy of the slightest interruption of the flow of
blood, the entrance of air, etc. Even as it is, the need of a
constant supply of oxygen in warm-blooded animals is much
greater than in cold-blooded creatures, which can long endure
almost entire cessation of both respiration and circulation,
owing to the comparatively slow rate of speed of the vital
machinery.
If one were to rely on mere appearances he might suppose
that in the more active condition of certain organs there was
less chemical interchange (respiration) between the blood aud
the tissues than in the resting stage, or, properly speaking,
more tranquil stage, for it must be borne in mind that a living
cell is never wholly at rest ; its molecular changes are cease-
less. It happens, e. g., that when certain glands (salivary) are
secreting actively, the blood flowing from them is less venous
in appearance than when not functionally active. This is not
because less oxygen is used or less abstracted from the blood,
but because of the greatly increased speed of the blood-flow, so
that the total supply to draw from is so much larger that,
though more oxygen is actually used, it is not so much missed,
nor do the greater additions of carbon dioxide so rapidly pol-
lute this rapid stream.
It is thus seen that throughout the animal kingdom respira-
tion is fundamentally the same process. It is in every case
finally a consumption of oxygen and production of carbonic
anhydride by the individual cell, whether that be an Amoeba
or an element of man's brain. These are, however, but the
beginning and end of a very complicated biological history of
by far the greater part of which nothing is yet known ; and it
must be admitted that diffusion or any physical explanation
carries us but a little way on toward the understanding of it.
THE RESPIRATORY SYSTEM. 393
THE NERVOUS SYSTEM IN RELATION TO RESPIRA-
TION.
We have considered the muscular movements by which the
air is made to enter and leave the lungs in consequence of
changes in the diameters of the air-inclosing case, the thorax.
It remains to examine into the means by which these muscles
were set into harmonious action so as to accomplish the pur-
pose. The nerves supplying the muscles of respiration are de-
rived from the spinal cord, so that they must be under the do-
minion of central nerve-cells situated either in the cord or the
brain. Is the influence that proceeds outward generated within
the cells independently of any afferent impulses, or is it depend-
ent on such causes ?
A host of facts, experimental and other, show that the cen-
tral impulses are modified by afferent impulses reaching the
center through appropriate nerves. Moreover, drugs seem to
act directly on the center through the blood.
The vagus is without doubt the afferent respiratory nerve,
though how it is affected, whether by the mechanical movement
of the lungs, merely, by the condition of the blood as regards
its contained gases, or, as seems most likely, by a combination
of circumstances into which these enter and are probably the
principal, is not demonstrably clear. When others function as
afferent nerves, capable of modifying the action of the respira-
tory center, they are probably influenced by the respiratory
condition of the blood, though not necessarily exclusively.
But when all the principal afferent impulses are cut off by
division of the nerves reaching the respiratory center directly
or indirectly, respiration will still continue, provided the motor
nerves and the medulla remain intact.
The center, then, is not mainly at least, a reflex but an auto-
matic one, though its action is modified by afferent impulses
reaching it from every quarter.
It has been argued that there are both inspiratory and ex-
piratory centers in the spinal cord, but this can not yet be re-
garded as established. But, as we have pointed out, on more
than one occasion, we must always be on our guard in inter-
preting the behavior of one part when another is out of gear.
The Influence of the Condition of the Blood in Respiration. —
If for any reason the tissues are not receiving a due supply of
oxygen, they manifest their disapproval, to speak figuratively,
394
COMPARATIVE PHYSIOLOGY.
Brain above medulla from which
impulses modifying respiration
may proceed.
'acial muscles.
Respiratory centre
in the medulla.
Cutaneous surface from which
afferent impulses proceed cZn
rectly to brain.
---I — Thoracic resp. muscles.
Spinal cord.—
r— Respiratory tract.
Diaphragm with
phrenic nerve.
Cutaneous sur-
face from which
impulses reach res-
piratory centre by
spinal cord.
Fig. 809.— Diagram intended to illustrate nervous mechanism of respiration. Arrows
indicate course of impulses.
THE RESPIRATORY SYSTEM. 395
by reports to the responsible center in the medulla, and if the
medulla is a sharer in the lack, as it naturally would be, it takes
action independently. One of the most obvious instances in
which there is oxygen starvation is when there is hindrance to
the entrance of air, owing to obstruction in the respiratory tract.
At first the breathing is merely accelerated, with perhaps
some increase in the depth of the inspirations (hyperpnoea), a
stage which is soon succeeded by labored breathing (dyspnoea),
which, after the medulla has called all the muscles usually em-
ployed in respiration into violent action, passes into convul-
sions, in which every muscle may take part.
In other words, the respiratory impulses not only pass along
their usual paths as energetically as possible, but radiate into
unusual ones and pass by nerves not commonly thus set into
functional activity.
It would be more correct, perhaps, to assume that the vari-
ous parts of the nervous system are so linked together that ex-
cessive activity of one set of connections acts like a stimulus to
rouse another set into action, the order in which this happens
depending on the law of habit — habit personal and especially
ancestral. An opposite condition to that described, known as
apnoea, may be induced by pumping air into an amimal's chest
very rapidly by a bellows ; or in one's self by a succession of
rapid, deep respirations.
After ceasing, the breathing may be entirely interrupted
for a brief interval, then commence very quietly, gradually in-
creasing to the normal.
Apnoea has been interpreted in two ways. Some think that
it is due to fatigue of the muscles of respiration or the respira-
tory center; others that the blood has under these circum-
stances an excess of oxygen, which so influences the respiratory
center that it is quieted (inhibited) for a time.
The latter view is that usualty adopted ; but considering that
apnoea results from the sobbing of children following a pro-
longed fit of crying, also in Cheyne-Stokes and other abnormal
forms of breathing, and that the blood is normally almost satu-
rated with oxygen, it will be agreed that there is a good deal to
be said for the first view, especially that part of it which repre-
sents the cessation of breathing as owing to excessive activity
and exhaustion of the respiratory center. We find such a calm
in asphyxia after the convulsive storm. Perhaps if we regard
the respiratory center as double, half being situated on each side
396 COMPARATIVE PHYSIOLOGY.
of the middle Hue ; also as made up of au inspiratory and ex-
piratory part; automatic essentially, but greatly modified by
afferent impulses, especially those ascending the vagi nerves;
while the latter may be considered as containing both inhib-
itory and augmenting fibers for the center, the whole process
will be clearer. Respiration on this view would be self -regula-
tive ; the deeper the inspiration, the stronger the inhibitory in-
fluence, so the greater the tendency to arrest of inspiration;
hence either expiration or apncea.
Is it, then, the excessive accumulation of carbon dioxide or
the deficiency of oxygen that induces dyspnoea ? Considering
that the former gas acts as a narcotic, and does not induce con-
vulsions, even when it constitutes a large percentage of the
atmosphere breathed, and that the need of oxygen for the tis-
sues is constant, it certainly seems most reasonable to conclude
that the phenomena of dyspnoea are owing to the lack of oxy-
gen, chiefly at least; though the presence of an excess of car-
bonic anhydride may take some share in arousing that vigorous
effort on the part of the nervous system, to restore the func-
tional equilibrium, so evident under the circumstances.
THE INFLUENCE OF RESPIRATION ON THE
CIRCULATION.
An examination of tracings of the intra-thoracic and blood-
pressure, taken simultaneously, shows (1) that during inspira-
tion the blood-pressure rises and the intra-thoracic pressure
falls ; (2) that during expiration the reverse is true ; and (3)
that the heart-beat is slowed, and has a decided effect on the
form of the pulse. But it also appears that the period of high-
est blood-pressure is just after expiration has begun.
We must now attempt to explain how these changes are
brought about. By intra-thoracic pressure is meant the press-
ure the lungs exert on the costal pleura or any organ within
the chest, which must differ from intra-pulmonary pressure
and the pressure of the atmosphere, because of the resistance of
the lungs by virtue of their own elasticity.
It has been noted that even in death the lungs remain par-
tially distended ; and that when the thorax is opened the pul-
monary collapse wbich follows demonstrates that their elas-
ticity amounts to about five millimetres of mercury, which
must, of course, represent but a small portion of that elasticity
THE RESPIRATORY SYSTEM.
397
which may he brought into play when these organs are greatly
distended, so that they never press on the costal walls, heart,
Fig. 310. — Tracings of blood-pressure and intrathoracic pressure in the dog (after
Foster), a, blood-pressure tracing showing irregularities due to respiration and
pulse; b, curve of intrathoracic pressure; i, beginning of inspiration; e, of expira-
tion. Intrathoracic pressure is seen to rise rapidly after inspiration ceases, and
then slowly sinks as the expiratory blast continues, to become a rapid fall when
inspiration begins.
etc., with a pressure equal to that of the atmosphere. It follows
that the deeper the inspiration the greater the difference be-
tween the intra-thoracic and the atmospheric pressure. Even
in expiration, except when forced, the intra-thoracic pressure
remains less, for the same reason.
These conditions must have an influence on the heart and
blood-vessels. Bearing in mind that the pressure without is
practically constant and always greater than that within the
thorax, the conditions are favorable to the flow of blood toward
the heart. As in inspiration, the pressure on the great veins
and the heart is diminished, and, as these organs are not rigid,
they tend to expand within the thorax, thus favoring an on-
ward flow. But the opposite effect would follow as regards the
large arteries. Their expansion must tend to withdraw blood.
During expiration the conditions are reversed. The effects on
the great veins can be observed by laying them bare in the
neck of an animal, when it may be seen that during inspiration
they become partially collapsed, and refilled during expiration.
In consequence of the marked thickness of the coats of the
great arteries, the effect of changes in intra-thoracic pressure
must be slight. The comparatively thin-walled auricles act
somewhat as the veins, and it is likely that the increase of
pressure during expiration must favor, so far as it goes, the car-
diac systole.
398 COMPARATIVE PHYSIOLOGY.
More blood, then, entering the right side of the heart dur-
ing inspiration, more will be thrown into tbe systemic circula-
tion, unless it be retained in the lungs, and, unless the effect be
counteracted, the arterial pressure will rise, and, as all the con-
ditions are reversed during expiration, we look for and find
exactly opposite results. The lungs themselves, however, must
be taken into the account. During inspiration room is pro-
vided for an increased quantity of blood, the resistance to its
flow is lessened, hence more blood reaches the left side of the
heart. The immediate effect would be, notwithstanding, some
diminution in the quantity flowing to the left heart, in conse-
quence of the sudden widening of the pulmonary vessels, the
reverse of which would follow during expiration ; hence the
period of highest intra-thoracic pressure is after the onset of
the expiratory act. During inspiration the descent of the
diaphragm compressing the abdominal organs is thought to
force on blood from the abdominal veins into the thoracic vena
cava.
That the respiratory movements do exert in some way a
pronounced effect on the circulation the student may demon-
strate to himself in the following ways : 1. After a full inspira-
tion, close the glottis and attempt to expire forcibly, keeping
the fingers on the radial artery. It may be noticed that the
pulse is modified or possibly for a moment disappears. 2. Re-
verse the experiment by trying to inspire forcibly with closed
glottis after a strong expiration, when the pulse will again be
found to vary. In the first instance, the heart is comparatively
empty and hampered in its action, intra-thoracic pressure being
so great as to prevent the entrance of venous blood by com-
pression of the heart and veins, while that already within the
organ and returning to it from the lungs soon passes on into
the general system, hence the pulseless condition. The expla-
nation is to be reversed for the second case. The heart's beat is
modified, probably reflexly, through the cardio-inhibitory cen-
ter, for the changes in the pulse-rate do not occur when the vagi
nerves are cut, at least not to nearly the same extent.
Comparative.— It may be stated that the cardiac phenomena
referred to in this section are much more marked in some ani-
mals than in others. Very little change may be observed in
the pulse-rate in man, while in the dog it is so decided that one
observing it for the first time might suppose that such pro-
nounced irregularity of the heart was the result of disease ;
THE RESPIRATORY SYSTEM. 399
though even in this animal there are variations in this respect
with the breed, age, etc.
The Respiration and Circulation in Asphyxia.— A most in-
structive experiment may be arranged thus :
Let an anaesthetized rabbit, cat, or such-like animal, have
the carotid of one side connected with a glass tube as before
desci'ibed (pages 228, 229), by which the blood-pressure and its
changes may be indicated, and, when the normal respiratory
acts have been carefully observed, proceed to notice the effects
on the blood-pressure, etc., of pumping air into the chest by a
bellows, of hindering the ingress of air to a moderate degree,
and of struggling. "With a small animal it will be difficult to
observe the respiratory effects on the blood-pressure by simply
watching the oscillations of the fluid in the glass tube, but this
is readily enough made out if more elaborate arrangements be
made, so that a graphic tracing may be obtained.
But the main events of asphyxia may be well (perhaps best)
studied in this manner:
Let the trachea be occluded (ligatured). At once the blood-
pressure will be seen to rise and remain elevated for some time,
then gradually fall to zero. These changes are contemporane-
ous with a series of remarkable manifestations of disturbance
in the respiratory system as it at first appears, but in reality
due to wide-spread and profound nutritive disturbance. So far
as the breathing is concerned, it may be seen to become more
rapid, deeper, and labored, in which the expiratory phase be-
comes more than proportionably marked (dyspnoea) ; this is fol-
lowed by the gradual action of other muscles than those usually
employed in respiration, until the whole body passes into a ter-
rible convulsion — a muscle-storm in consequence of a nerve-
storm. When this has lasted a variable time, but usually
about one minute, there follows a period of exhaustion, during
which the subject of the experiment is in a motionless condi-
tion, interrupted by an occasional respiration, in which inspi-
ration is more pronounced than expiration; and, finally, the
animal quietly stretches every limb, the sphincters are relaxed,
there may be a discharge of urine or faeces from peristaltic
movements of the bladder or intestines, and death ends a strik-
ing scene. These events may be classified in three stages,
though the first and second especially merge into one another:
1. Stage of dyspnoea. 2. Stage of convulsions. 3. Stage of
exhaustion.
400 COMPARATIVE PHYSIOLOGY. •
It is during the first two stages that the blood-pressui'e rises,
and during the third that it sinks, due in the first instance
chiefly to excessive activity of the vaso-motor center, and in
the second to its exhaustion and the weakening of the heart-
beat.
These violent movements are owing, we repeat, to the action
of blood deficient in oxygen on the respiratory center (or cen-
ters), leading to inordinate action followed by exhaustion.
The duration of the stages of asphyxia varies with the ani-
mal, but rarely exceeds five minutes. In this connection it may
be noted that newly born animals (kittens, puppies) bear im-
mersion in water for as much as from thirty to fifty minutes,
while an adult dog dies within four or five minutes. This is
to be explained by the feeble metabolism of new-born mam-
mals, which so slowly uses up the vital air (oxygen).
If the chest of an animal be opened, though the respiratory
muscles contract as usual there is, of course, no ventilation of
the lungs which lie collapsed in the chest ; and the animal dies
about as quickly as if its trachea were occluded. It passes
through all the phases of asphyxia as in the former case ; but
additional information may be gained. The heart is seen to
beat at first more quickly and forcibly, later vigorously though
slower, and finally both feebly and irregularly, till the ventri-
cles, then the left auricle, and finally tho right auricle cease to
beat at all or only at long intervals. The terminations of the
great veins (representing the sinus venosus) beat last of all.
At death the heart and great veins are much distended
with blood, the arteries comparatively empty. Even after
rigor mortis has set in, the right heart is still much engorged.
These phenomena are the result of the operation of several
causes. The increasingly venous blood at first stimulates the
heart probably directly, in part at least, but later has the con-
trary effect. The nutrition of the organ suffers from the de-
graded blood, from which it must needs derive its supplies.
The cardio-inhibitory center probably has a large share in the
slowing of the heart, if not also in quickening it. Whether
the accelerator fibers of the vagus or sympathetic play any
part is uncertain. The increase of peripheral resistance caused
by the action of the vaso-motor center makes it more difficult
for the heart to empty its left side and thus receive the venous
blood as it pours on. At the same time the deep inspirations
(when the chest is unopened) favor the onflow of venous blood;
THE RESPIRATORY SYSTEM. 401
and in any case the whole venous system, including the right
heart, tends to become engorged from these several causes act-
ing together. The heart gives up the struggle, unable to main-
tain it, but not so long as it can beat in any part.
The share which the elasticity of the arteries takes in
forcing on the blood when the heart ceases, and the contraction
of the muscular coat of these vessels, especially the smaller,
must not be left out of the account in explaining the phenom-
ena of asphyxia and the post-mortem appearances.
Pathological.— The importance of being practically as well
as theoretically acquainted with the facts of asphyxia is very
great.
The appearance of the heart and venous system gives une-
quivocal evidence as to the mode of death in any case of as-
phyxia; and the contrast between the heart of an animal bled
to death, or that has died of a lingering disease, and one
drowned, hanged, or otherwise asphyxiated, is extreme.
We strongly recommend the student to asphyxiate some
small mammal placed under the influence of an anaesthetic,
and to note the phenomena, preferably with the chest opened ;
and to follow up these observations by others after the onset of
rigor mortis.
PECULIAR RESPIRATORY MOVEMENTS.
Though at first sight these seem so different, and are so as
regards acts of expression, yet from the respiratory point of
view they resemble each other closely; they are all reflex, and,
of course, involuntary. Many of them have a common pur-
pose, either the better to ventilate the lungs, to clear them of
foreign bodies, or to prevent their ingress.
Coughing, in which such a purpose is evident, is made up of
several expiratory efforts preceded by an inspiratory act. The
afferent nerve is usually the vagus or laryngeal, but may be one
or more of several others.
The glottis presents characteristic appearances, being closed
and then opened suddenly, the mouth being kept open.
Coughing is often induced in attempting to examine the ear
with instruments. (Reflex act).
Laughing is very similar to the last, so far as the behavior
of the glottis is concerned, though it usually acts more rapidly,
of course. Several expirations follow a deep inspiration.
26
402 COMPARATIVE PHYSIOLOGY.
Crying is essentially the same as laughing, hut the facial
expression is different, and the lachrymal gland functions ex-
cessively, though with some persons this occurs during laughter
also.
Sobbing is made up of a series of inspirations, in which the
glottis is partially closed, followed by a deep expiration.
Yawning involves a deep-drawn, slow inspiration, followed
by a more sudden expiration, with a well-known depression of
the lower jaw and usually stretching movements.
Sighing is much like the preceding, though the mouth is not
opened widely if at all, nor do the stretching movements com-
monly occur.
Hiccough is produced by a sudden inspiratory effort, though
fruitless, inasmuch as the glottis is suddenly closed. It is
spoken of as spasm of the diaphragm, and when long continued
is very exhaustive.
Sneezing is the result of a powerful and sudden expiratory
act following a deep inspiration, the mouth being usually closed
by the anterior pillars of the fauces against the outgoing cur-
rent of ah", which then makes its exit through the nose, while
the glottis is forcibly opened after sudden closure. It will be
noticed that in most of these acts the glottis is momentarily
closed, which is never the case in mammals during quiet res-
piration.
This temporary occlusion of the respiratory passages per-
mits of a higher intrapulmonary pressure, which is very effect-
ive in clearing the passages of excess of mucus, etc., when the
glottis is suddenly opened. Though the acts described are all
involuntary, they may most of them be imitated and thus
studied deliberately by the student. It will also appear, con-
sidering the many ways in which some if not all of them may
be brought about, that if the medullary center is responsible for
the initiation of them it must be accessible by numberless paths.
Comparative. — Few of the lower animals cough with the
same facility as man, while laughing is all but unknown, cry-
ing and sobbing rare, though the whining of dogs is allied to
the crying of human beings.
Sneezing seems to be voluntary in some animals, as squir-
rels, when engaged in toilet operations, etc.
Barking is voluntary, and in mechanism resembles cough-
ing, the vocal cords being, however, more definitely employed,
as also in growling.
THE RESPIRATORY SYSTEM. 403
Balding, neighing, braying, etc., are made up of long ex-
piratory acts, preceded by one or more inspirations. The vocal
cords are also rendered tense.
SPECIAL CONSIDERATIONS.
Pathological and Clinical. —The number of diseases that lessen
the amount of available pulmonary tissue, or hamper the move-
ments of the chest, are many, and only the briefest reference
can be made to a few of them.
Inflammation of the lungs may render a greater or less por-
tion of one or both lungs solid; inflammation of the pleura
(pleuritis, pleurisy) by the dryness, pain, etc., may restrict the
thoracic movements; phthisis may solidify or excavate the
lungs, or by pleuritic inflammation glue the costal and pulmo-
nary pleural surfaces together; bronchitis may clog the tubes
and other air-passages with altered secretions ; emphysema (dis-
tention of air-cells) may destroy elasticity of parts of the lung ;
pneuma-thorax from rupture of the lung-tissue and consequent
accumulation of gases in the pleural cavity, or pleurisy with
effusion render one lung all but useless from pressure. In all
such cases Nature attempts to make up what is lost in amplitude
by increase in rapidity of the respiratory movements. It is
interesting to note too how the other lung, in diseased condi-
tions, if it remain unaffected, enlarges to compensate for the
loss on the opposite side. When the muscles are weak, espe-
cially if there be hindrance to the entrance of air while the
thoracic movements are marked, there may be bulging inward
of the intercostal spaces.
Normally, this would also occur, as the intra-thoracic press-
ure is less than the atmospheric, were it not for the fact that the
intercostal muscles when contracting have a certain resisting
power.
The imperfect respiration of animals when dying, permitting
the accumulation of carbonic anhydride with its soporific
effects, smooths the way leading to the end ; so that there may
be to the uninitiated the appearance of a suffering which does
not exist, consciousness itself being either wholly or partially
absent. The dyspnoea of anaemic animals, whether from sud-
den loss of blood or from imperfect renewal of the haemo-
globin, shows that this substance has a respiratory function;
while in forms of cardiac disease with regurgitation, etc., the
404 COMPARATIVE PHYSIOLOGY.
blood may be imperfectly oxidized, giving rise to labored res-
piration.
Personal Observation. — As hinted from time to time during
the treatment of this subject, there is a large number of facts
the student may verify for himself.
A simple way of proving that CO2 is exhaled is to breathe
(blow) into a vessel containing some clear solution of quick-
lime (CaO), the turbidity showing that an insoluble salt of lime
(CaC03) has been formed by the addition of this gas.
The functions of most of the respiratory muscles, the phe-
nomena of dyspnoea, apncea (by a series of long breaths), par-
tial asphyxia by holding the breath, and many other experi-
ments, simple but convincing, will occur to the student who is
willing to learn in this way.
The observation of respiration in a dreaming animal (dog)
will show how mental occurrences affect the respiratory center
in the absence of all the usual outward influences. The respira-
tion of the domestic animals, and of the frog, turtle, snake, and
fish, is easily watched if these cold-blooded animals be placed
for observation beneath a glass vessel. Their study will teach
how manifold are the ways by which the one end is attained.
Compare the tracings of Fig. 305.
Evolution. — A study of embryology shows that the respira-
tory and circulatory systems develop together ; that the vascu-
lar system functions largely as a respiratory system also in cer.
tain stages, and remains such, from a physiological point of
view, throughout embryonic life.
The changes that take place in the vascular system — the
heart, especially — of the mammal when the lungs have become
functionally active at birth, show how one set of organs modi-
fies the other.
When one considers, in addition to these facts, that the
digestive as well as the vascular and respiratory organs are
represented in one group of structures in a jelly-fish, and that
the lungs of the mammal are derived from the same mesoblast
as gives rise to the digestive and circulatory organs, many of
the relations of these systems in the highest groups of animals
become intelligible ; but unless there be descent with modifica-
tion, these facts, clear enough from an evolutionary standpoint,
are isolated and out of joint, bound together by no common
principle that satisfies a philosophical biology.
It has been found that in hunting-dogs and wild rabbits the
THE RESPIRATORY SYSTEM. 405
vagus is more efficient than in other races of dogs and in rab-
bits kept in confinement ; and possibly this may in part account
for the greater speed and especially the endurance of the
former. The very conformation of some animals, as the grey-
hound, with his deep chest and capacious lungs, indicates an
unusual respiratory capacity.
The law of habit is well illustrated in the case of divers, who
can bear deprivation of air longer than those unaccustomed to
such submersion in water. Greater toleration on the part of
the respiratory center has probably much to do with the case,
though doubtless many other departures from the normal occur,
either independently or correlated to the changes in the respira-
tory center. Some mammals, like the whale, can long remain
under water.
Summary of the Physiology of Respiration.— The purpose
of respiration in all animals is to furnish oxygen for the tissues
and remove the carbonic anhydride they produce, which in all
vertebrates is accomplished by the exposure of the blood in
capillaries to the atmospheric air, either free or dissolved in
water. A membrane lined with cells always intervenes between
the capillaries and the air.
The air may be pumped in and out, or sucked in and forced
out.
Respiration in the Mammal.— The air enters the lungs,
owing to the enlargement of the chest in three directions by the
action of certain muscles. It leaves the lungs because of their
own elastic recoil and that of the chest- wall chiefly. Inspiration
is active, expiration chiefly passive.
The diaphragm is the principal muscle of respiration. In
some animals there is a well-marked facial and laryngeal as
well as thoracic respiration. Respiration is rhythmical, con-
sisting of inspiration, succeeded without appreciable pause by
expiration, the latter being in health of only slightly longer
duration. There is also a definite relation between the number
of respirations and of heart-beats. According as respiration is
normal, hurried, labored, or interrupted, we describe it as
eupnoe, hyperpnoea, dyspnoea, and apnoea. The intra-thoracic
pressure is never equal to the atmospheric — i. e., it is always
negative — except in forced expiration ; and the lungs are never
collapsed so long as the chest is unopened. The expired air
diffei'S from that inspired in being of the temperature of the
body, saturated with moisture, and containing about 4 to 5 per
406 COMPARATIVE PHYSIOLOGY.
cent less oxygen and 4 per cent more carbonic anhydride, be-
sides certain indifferently known bodies, the result of tissue
metabolism, excreted by the lungs.
The quantity of air actually moved by a respiratory act, as
compared with the total capacity of the respiratory organs, is
small ; hence a great part must be played by diffusion. The
portion of air that can not be removed from the lungs by any
respiratory effort is relatively large.
It is customary to distinguish tidal, complementary, supple-
mentary, and residual air.
The vital capacity is estimated by the quantity of air the
respiratory organs can move, and is very variable.
The blood is the respiratory tissue, through the mediation
of its red cells, by the haemoglobin they contain. This sub-
stance is a ferruginous proteid, capable of crystallization, and
assuming under chemical treatment many modifications. When
it contains all the oxygen it can retain, it is said to be saturated
and is called oxy-haemoglobin, in which form it exists (with
some reduced haemoglobin) in arterial blood, and to a lesser
extent in venous blood, which differs from arterial in the rela-
tive proportions of haemoglobin (reduced) it contains, as viewed
from the respiratory standpoint.
Oxy-haemoglobin does not assume or part with its oxygen,
according to the Henry -Dalton law of pressures, nor is this gas
in a state of ordinary chemical combination. It is found that
the oxygen tension of the blood is lower and that of carbonic
anhydride higher than in the air of the alveoli of the lungs,
while the same may be said of the tissues and the blood re-
spectively. This has been, however, recently again denied.
Respiration is a vital process, though certain physical con-
ditions (temperature and pressure) must be rigidly maintained
in order that the gaseous interchanges shall take place. Res-
piration is always fundamentally bound up with the metabo-
lism of the tissues themselves. All animal cells, whether they
exist as unicellular animals (Amoeba) or as the components of
complex organs, use np oxygen and produce carbonic dioxide.
Respiratory organs, usually so called, and the respiratory tissue
par excellence (the blood) are only supplementary mechanisms
to facilitate tissue respiration. Carbonic anhydride exists in
blood probably in combination with sodium salts, though the
whole matter is very obscure.
Respiration, like all the other functions of the body, is con-
THE RESPIRATORY SYSTEM. 4Q7
trolled by the central nervous system through nerves. The
medulla oblongata is chiefly concerned, and especially one
small part of it known as the respiratory center. It is possible,
even probable, that there are subordinate centers in the cord,
which, under peculiar circumstances, assume importance ; but
how far they act in concert with the medullary center, or
whether they act at all when normal conditions prevail, is an
open question.
The vagus is the principal afferent respiratory nerve. The
efferent nerves are the phrenies. intercostals, and others supply-
ing the various muscles used in moving the chest-walls, etc.
The respiratory center is automatic, but its action is sus-
ceptible of modification through afferent influences taking a
variety of paths, the principal of which is along the vagi nerves.
The respiratory, vaso-motor, and cardio-inhibitory centers seem
to act somewhat in concert.
Blood-pressure is being constantly modified by the respira-
tory act, rising with inspiration and sinking with expiration.
In some animals the heart-beat also varies with these phases of
respiration, becoming slow and irregular during expiration.
Into the causation of these changes both mechanical and nerv-
ous factors enter, and make a very complex mesh, which we
can at present but imperfectly unravel. When the access of
air to the tissues is prevented, a series of stages of respiratory
activity and decline, accompanied by pronounced changes in
the vascular system, are passed through, known as asphyxia.
Three stages are distinguishable : one of dyspnoea, one of
convulsions, and one of exhaustion — while at the same time
there is a rise of blood-pressure during the first two, and a
decline during the third, accompanied by marked alterations in
the cardiac rhythm.
PROTECTIVE AND EXCRETORY FUNCTIONS
OF THE SKIN.
As lias been intimated from time to time, thus far, as a
result of the metabolism of the tissues, certain products require
constant removal from the blood to prevent poisonous effects.
These substances are in all probability much more numerous
than physiological chemistry has as yet distinctly recognized
or, at all events, isolated. Quantitatively considered, the most
important are carbonic anhydride, water, urea, and, of less im-
portance, perhaps, certain salts.
In many invertebrates and in all vertebrates several organs
take part in this work of elimination of waste products or puri-
fication of the blood, one set of which — the respiratory — we
have just studied ; and we now continue the consideration of
the subject of excretion, this term being reserved for the pro-
cess of separating harmful products from the blood and dis-
charging them from the body.
We strongly recommend the student to make the study of
excretion comparative in the sense of noting how one organ
engaged in the process supplements another. A clear under-
standing of this relation even to details makes the practice of
medicine more scientific and practically effective, and gives
physiology greater breadth.
The skin has a triple function : it is protective, excretory,
sensory, and, we may add, nutritive (absorptive) and respira-
tory, especially in some groups of animals.
As a sensory organ, the skin will receive attention later.
Protective Function of the Skin. — Comparative. — Among
many groups of invertebrates the principal use of the exterior
covering of the body is manifestly protection. Among these
forms, an internal skeleton being absent, the exo-skeleton is
developed externally, and serves not only for protection, but
for the attachment of muscles, as seen in crustaceans and in-
PROTECTIVE FUNCTION OP THE SKIN.
409
sects. But this part of the subject is too large for detailed
treatment in such a work as this. Turning to the vertebrates,
we see scales, bony plates, feathers, spines, hair, etc., most of
them to be regarded as modifications of the epidermis, always
useful, and frequently also ornamental.
Primitive man was probably much more hirsute than his
modern representative ; and, though the human subject is at
present provided with a skin in which protective functions are
at their lowest, still the epidermis does serve such a purpose, as
all have some time realized when
it has been accidentally removed
by blistering, etc.
Taking the structure of the
skin of man as representing that
of mammals generally, certain
points claim attention from the
physiologist. Its elasticity, the
failure of which in old age ac-
counts for wrinkles; its epider-
E>
■jrftSfea^y •"?}..- -*3?' ,v^
Fig. 311.
Fig. 312.
Fig. 311.— Sudoriparous gland?. 1x20. (Af ter Sappey.) 1. 1. epidermis; 2. 2, mucous
layer; 3, 3, papillae; 4. 4, derma; 5. 5, subcutaneous areola tissue; 6. 0. 6, 6, sudo-
riparous glands; 7 7, adipose vesicles; 8, 8, excretory ducts in derma; 9, 9, excre-
tory ducts divided.
Fig. 312. — Portion of skin of palm of hand about one-half an inch (127 mm.) square.
1x4. (After Sappey.) 1, 1, 1, 1, openings of sudoriferous ducts; 2, 2, 2, 2, grooves
between papillse of skin.
mal covering, made up of numerous layers of cells; its coiled
and spirally twisted sudoriferous glands, permitting of move-
ments of the skin without harm to these structures; its hair-
follicles and associated sebaceous glands, the fatty secretion of
which keeps the hair and the skin generally soft and pliable.
410
COMPARATIVE PHYSIOLOGY.
Tlie muscles of the skin, which either move it as a whole or
erect individual hairs, play an important part in. modifying" ex-
pression, well seen in the whole canine tribe and many others.
There are several
modifications of the
sebaceous glands that
furnish highly odor-
iferous secretions as
in the civet cat, the
skunk, the musk-
deer, and many low-
er vertebrates. In
some, these are pro-
tective (skunk) ; in
others, though they
may not be agreeable
to the senses of man,
they are doubtless at-
tractive to the fe-
males of the same
tribe, and are to be
regarded as impor-
tant in " sexual se-
lection," being often
confined to the males
alone.
Ear-wax and the
Meibomian secretion
are the work of
modified sebaceous
glands ; as also the
oil-glands so highly
developed in birds,
especially aquatic
forms, and of which
these creatures make
great use in preserv-
Fig. 313.— Hair and hair-follicle (after Sappey). 1, root ing their feathers
of hair; 2, bulb of hair; 3, internal root-sheath; 5, from wettin0-
membrane of hair-follicle; 6, external membrane of &
follicle; 7, 7, muscular bands attached to follicle; In our ClomOSUC
8, 8, extremities of bands passing to skin; 9, com- . ™„,T ac!
pound sebaceous Kiand, with duct (10) opening into animals we may es-
W^gd$^^'*amnB*"i pecially notice a cu
THE EXCRETORY FUNCTION OF THE SKIN. 4H
taneous gland in the pig, placed at the posterior inner aspect of
the knee and of considerable size.
In the sheep, the interungulate
gland is an inversion of the integu-
ment forming an elongated sac,
which is supplied with secreting
structures analogous to the seba-
ceous glands. The importance of
protective structures of this kind
in such situations is obvious.
THE EXCRETORY FUNCTION
OF THE SKIN.
The quantity of matter dis- Fig- 314. — Interungulate gland of
, , ., i,i i • • i sheep (Chauveau). «, inner as-
charged through the skm is large pectof first phalanx; b, hoof or
-greater in man than by the lungs Se Viteffi"1^ *tad; *
(about as 7 to 11), though the
amount is very variable, depending on the degree of activ-
ity of other related excreting organs, as the lungs and kidneys,
and largely upon the temperature as a physical condition; and
so in other animals.
When the watery vapor is carried off, before it can condense,
the perspiration is said to be insensible ; when small droplets
become visible, sensible. As to whether the one or the other is
predominant will, of course, depend on the rapidity of renewal
of the air, its humidity, and its temperature. Apart from the
temperature, the amount of sweat is influenced by the quality
and quantity of food and, especially of drink taken, the amount
of exercise, and psychic conditions ; not to speak of the effect of
drugs, poisons, or disease.
Perspiration in man is a clear fluid, mostly colorless, with
a characteristic odor, devoid of morphological elements (ex-
cept epidermal scales), and alkaline in reaction. It may be
acid from the admixture of the secretion of the sebaceous
glands.
Its solids (less than 2 per cent) consist of sodium salts,
mostly chlorides, cholesterin, neutral fats, and traces of urea.
The acids of the sweat belong to the fatty series (acetic, butyric,
formic, propionic, caprylic, cax^roic, etc.).
Pathological. — The sweat may contain blood, proteids, abun-
dance of urea (in cholera), uric acid, oxalates, sugar, lactic acid,
412 COMPARATIVE PHYSIOLOGY.
bile, indigo, and other pigments. Many medicines are elimi-
nated in part through the skin.
Respiration by the Skin.— Comparative.— In i-eptiles and
batrachians, with smooth, moist skin, the respiratory functions
of this organ are of great importance ; hence these animals can
live long under water.
It is estimated that in the frog the greater part of the car-
bonic anhydride of the body -waste is eliminated by the skin.
Certainly frogs can live for days immersed in a tank supplied
with running water ; and it is a significant fact that in this
animal the vessel that gives rise to the pulmonary artery sup-
plies also a cutaneous branch.
The respiratory capacity of the skin in man and most
mammals is comparatively small under ordinary circum-
stances. The amount of carbonic anhydride thus eliminated
in twenty-four hours in man is estimated at not more than 10
grammes. It varies greatly, however, with temperature, exer-
cise, etc.
The skin is highly vascular in mammals, and its importance
as a heat regulator is thus very great.
When an animal is varnished over, its temperature rapidly
falls, though heat production is in excess. From the fact that
life may be prolonged by diminishing loss of heat through
wrapping up the animal in cotton-wool, it is inferred that
depression of the temperature is, at all events, one of the causes
of death. Though the subject is obscure, it is likely that the
retention of poisonous products so acts as to derange metabo-
lism, as well as poison directly, which might thus lead to the
disorganization of the machinery of life to the point of disrup-
tion or death. It is also possible that the reduction of the tem-
perature from dilatation of the cutaneous vessels may be so
great that the animal is cooled below that point at which the
vital functions can continue.
THE EXCRETION OF PERSPIRATION.
In secretion in the wider sense we find usually certain nerv-
ous and vascular effects associated. The vessels supplying the
gland are dilated during the most active phase, and at the same
time nervous impulses are conveyed to the secreting cells which
stimulate them to action. There is a certain proportion of
water given off by transpiration ; but the sweat, as a whole,
THE EXCRETION OP PERSPIRATION. 413
even the major part of the water, is a genuine secretion, the
result of the metaholism of the cells.
From experiments it is clear that nervous influences alone,
in the absence of any vascular changes, or in the total depriva-
tion of blood, suffice to induce the secretion of perspiration. If
the central stump of the divided sciatic be stimulated, sweating
of the other limbs follows, showing that perspiration may be a
reflex act. It is found that stimulation of the peripheral end of
the divided cervical sympathetic leads to sweating on the cor-
responding side of tbe face.
Sweating during dyspnoea and from fear, when the cutane-
ous surfaces are pale, as well as in the dying animal, shows also
the independent influence over the sudorific glands of the nerv-
ous system. Heat induces sweating by acting both reflexly and
directly on the sweat-centers we may suppose. Unilateral
sweating is known as a pathological as well as experimental
phenomenon. Perspiration may be either increased or dimin-
ished in paralyzed limbs, according to circumstances. It is
possible that there is a paralytic secretion of sweat as of saliva.
The subject is very intricate, and will be referred to again on
account of the light it throws on metabolic pi'ocesses generally.
Absorption by the skin in man and other mammals is, under
natural conditions probably very slight, as would be expected
when it is borne in mind that the true skin is covered by sev-
eral layers of cells, the outer of which are hardened.
Ointments may unquestionably be forced in by rubbing ;
and perhaps absorption may take place when an animal's tis-
sues are starving, and food can not be made available through
the usual channels. It is certain that abraded surfaces are a
source of danger, from affording a means of entrance for dis-
ease-producing substances or for germs.
Comparative. — It is usually stated in works on physiology
that the horse sweats profusely, the ox less so ; the pig in the
snout: and the dog, cat, rabbit, rat, and mouse, either not at all
or in the feet (between the toes) only. That a closer observa-
tion of these animals will convince any one that the latter
statements are not strictly correct, we have no doubt. These
animals, it is true, do not perspire sensibly to any great extent ;
but to maintain that their skin has no excretory function is an
error.
Summary.— The skin of the mammal has protective, sensory,
respiratory, and excretory functions. The respiratory are in-
414 COMPARATIVE PHYSIOLOGY.
significant under ordinary circumstances in this group, though
well marked in reptiles and especially in hatrachians (frog,
menobranchus). Sweating is probably dependent on the action
of centers situated in the brain and spinal cord, through nerves
that run generally in sympathetic tracts during some part of
their course. While the function of sweating may go on inde-
pendently of abundant blood-supply, it is usually associated
with increased vascularity.
Sweat contains a very small quantity of solids, is alkaline in
reaction when pure, but liable to be acid from the admixture of
sebaceous matter that has undergone decomposition. Sebum
consists chiefly of olein, palmitin, soaps, cholesterin, and ex-
tractives of little known composition. The salty taste of the
perspiration is due chiefly to sodium chloride, and its smell to
volatile fatty acids ; especially is this so of the sweat of certain
parts of the body of man and other mammals.
The functional activity of the skin varies with the tempera-
ture, moisture, etc., of the air and certain internal conditions;
especially is it important to remember that it is one of a series
of excretory organs which act in harmony to eliminate the
waste of the body, so that when one functions more the other
may and usually does function less.
The protective function of the skin and its modified epithe-
lium (hair, horns, nails, feathers, etc.) is in man slight, but very
important in many other vertebrates, among which provision
against undue loss of temperature is one of the most constantly
operative, and enables a vast number of groups of animals to
adapt successfully to their varying surroundings.
EXCEETION BY THE KIDNEY.
The kidney in man and other mammals may be described as
a very complex arrangement of tubes lined with many differ-
ent forms of secreting- cells, surrounded by a great mesh work of
capillaries, bound together by connective tissue, the quantity
Fig. 315.— Vertical longitudinal section of horse's kidney (Chauveau). a, cortical por-
tion; b, medullary portion; c, peripheral portion of latter; d, interior of pelvis;
d', d', arms of pelvis; e, border of crest; /, infundibulum; g, ureter.
varying with the animal, and the whole inclosed in a capsule.
The organ is well supplied with lymphatics and nerves.
Though the tubes are so complex, the kidney may be divided
into zones which contain mostly but one kind of tubule.
Among vertebrates, till the reptiles are reached, the kidney
is a persistent Wolffian body, hence its more simple form.
In most fishes the kidney is a very elongated organ, though
416
COMPARATIVE PHYSIOLOGY.
T?,n sip. stricture of kidnev (after Landois). I. Blood-vessels and tubes (semi-dia-
SimS A Cap Ss of cortical substance. B.. Capillaries of medu lary
gSSESS?" 1, ^impenetrating Malpighian body; 2, van _.„£ rom a Ma pig-
hem hortv It arterioles rectee: C, venm rectse 7, F, interlobular veins, ^.stciiaie
ve ns ) / cansu «•« of Mullur X, A', convoluted tubes; T, T, T, tubes of Henle;
N NNN ■ diiimui.i.-utiiiK tubes 0, 0, straight tubes; O, opening into pelvis of
kidney ' II. Malpighian body. A, artery; ti, vein; 0, capilfanes; if, epithelium
EXCRETION BY THE KIDNEY.
41?
of capsule; H, beginning of convoluted tube. III. Rodded cells from convoluted
tube. 1, view from surface; 2, side view {G, granular zone). IV. Cells lining
tubes of Henle. V. Cells lining communicating tubes. VI. Section of straight
tube.
in the lowest it consists of little more than tubules, coiling but
slightly, ending by one extremity in a glomerulus and by the
Fig. 317
• 317.— Blood-vessels of MalpigTuan bodies and convoluted tubes of kidney (after
Sappey). 1, 1, Malpighian bodies surrounded by capsules; 2. 2, 2. convoluted
tubes connected with Malpighian bodies; 3. artery branching to go to Malpighian
bodies; 4, 4, 4. branches of artery; 6, 6, Malpighian bodies from" which a portion
of capsules has been removed; 7. 7, 7, vessels passing out of Malpighian bodies; 8.
vessel, branches of which (9) pass to capillary plexus (10).
27
418
COMPARATIVE PHYSIOLOGY.
other opening- into a long- common efferent tube or duct. The
glomerulus is, however, peculiar to the vertebrate kidney.
The graded complexity in arrangement, etc., of the tubes is
represented well in the figure below. It is a significant fact
Pig. 318.— Diagrammatic representation of distribution of tubules of kidney (after
Huxley). 0, cortical region; B. boundary zone, containing large part of Henle's ,
loops; P, papillary zone, in which are the main outflow tubules.
that the kidney of the human subject is lobulated in the embryo,
which condition is persistent in some mammals (ruminants, etc.).
As the lungs are the organs employed especially for the
elimination of carbonic anhydride, so the kidneys are above
all others the excretors of the nitrogenous waste products of the
body chiefly in the form of uric acid or urea. Before treating of
secretion by the kidney it will be well to examine into the phys-
ical and chemical properties of urine with some detail, especially
on account of its great importance in the diagnosis of disease.
EXCRETION BY THE KIDNEY.
419
URINE CONSIDERED PHYSICALLY AND CHEMI-
CALLY.
Urine is naturally a fluid of very variable composition, espe-
cially regarded quantitatively — a fact to be borne in mind in
considering' all statements of the constitution of this fluid.
Specific Gravity. — Urine must needs be heavier than water,
on account of the large variety of solids it contains. The aver-
age specific gravity of the urine for the twenty -four hours is in
man 1015 to 1020 ; in the horse, 1030 to 1060 ; in the ox, 1020
to 1030 ; in the sheep and goat, 1005 to 1015 ; in the pig, 1010 to
1015 ; in the dog, 1030 to 1050. It is lowest in the morning and
varies greatly with the quantity and kind of food eaten, the ac-
tivity of the lungs and especially of the skin, etc.
Color. — Some shade of yellow, which is also very variable,
being increased in depth either by the presence of an excess of
pigment or a diminution of water. In herbivora the urine is
turbid, and may darken on exposure to the air.
The reaction of human urine is acid, owing to acid salts,
especially acid sodium phosphate (NafLPQ,). In the carnivora it
is strongly acid ; in the herbivora, alkaline. The reaction of urine
depends largely though not wholly on the character of the food.
Quantity. — This is, of course, like the specific gravity, highly
variable, and frequently they run parallel with each other.
The following tabular statement will prove useful for refer-
ence:
Composition of the Urine (Boussingault).
Horse.*
Cow.t
Pigt
Urea
31-0
4-7
20-1
15-5
4-2
10-8
1-2
0-7
1-0
o-o
910-0
18-5
16-5
17-2
16-1
4-7
06
3-6
1-5
trace.
o-o
921-3
4-9
Potassium hippurate
o-o
Alkaline lactates.
Potassium bicarbonate
10-7
Magnesium carbonate
0-9
Calcium carbonate
Potassium sulphate
20
Sodium chloride
1-3
01
1-0
Water and substances undetermined.
979-1
Total
1000-0
1000-0
1000-0
* Diet of clover, grass, and oats.
X Diet of potatoes, cooked.
f Diet of hay and potatoes.
420 COMPARATIVE PHYSIOLOGY.
Nitrogenous Crystalline Bodies.— These are the derivatives
of the metabolism of the body, and not in any appreciable de-
gree drawn froni the food itself. Besides urea, and of much less
importance, occurring in small quantities, are bodies that may
be regarded as less oxidized forms of nitrogenous metabolism,
such as creatinin, xanthin, hypoxanthin (sarkin), hippuric acid,
ammonium oxalurate, and urea, CO > tvttt2 The latter was
first prepared artificially from ammonium cyanate, |jtt I O,
with which it is isomeric. The quantity of urea is generally
in inverse proportion to that of hippuric acid, and varies much
with the diet in the herbivora. The richer in proteids the diet,
as when oats are fed, the greater the quantity of urea. In the
horse this proportion varies with the ordinary diet between
2*5 and 4"0 per cent.
When air has free access to urine for some time in a warm
room, the urea becomes ammonium carbonate by hydration,
probably owing to the influence of micro-organisms, thus:
CO (NH2)o+ 2 H,0 = (NH4)2 C03; hence the strong ammoniacal
smell of old urine, urinals, etc.
Uric acid (C5H4N4O3) occurs sparingly (see table), combined
with sodium and ammonium chiefly as acid salts.
Non-nitrogenous Organic Bodies.— A series of well-known
aromatic bodies occurs in urine, especially in that of the horse,
cow, etc. These are phenol, cresol, pyrocatechin, etc., which
occur not free, but united with sulphuric acid.
Inorganic Salts. — These are mostly in simple solution, in
urine, and not as in some other fluids of the body, united with
proteid bodies. The salts are chlorides, phosphates, sulphates,
nitrates, and carbonates ; the bases being sodium, potassium,
calcium, magnesium. The imosphates are to be traced to the
food, to the phosphorus of proteids, and to phosphorized fats
(lecithin). The sulphates are derived from those of the food
and from the sulphur of the proteids of the body. The greater
part of the carbonates is supplied directly by the food. In the
horse the salts of potassium and calcium (CaCOa), are abundant;
while in the dog magnesium and calcium salts abound as sul-
phates and phosphates.
Doubtless many bodies appear either regularly or occasion-
ally in urine that have so far escaped detection, which are, like
the poisonous exhalations of the lungs, not the less important
because unknown to science.
EXCRETION BY THE KIDNEY. 421
Abnormal Urine. — There is not a substance in the urine that
does not vary under disease, while the possible additions act-
ually known are legion. These may be derived either from
the blood or from the kidneys and other parts of the urinary
tract. The kidneys seem to take upon themselves more readily
than any other organ the duty of eliminating foreign matters
from the body. But this aspect of the subject is too wide for
detailed consideration in this work.
The student of medicine should be thoroughly familiar with
the urine in its normal condition before he enters upon the
examination of the variations produced by disease. This is not
difficult, and much of it may be carried out with but a meager
supply of apparatus. For this purpose, however, we recom-
mend some work devoted to the chemical and microscopic study
of the urine.
It greatly assists to remember a few points in regard to solu-
bilities. From a physiological point of view, the urine and its
variations, as the result of changes in the organism, may be ob-
served with advantage in one's own person — eg., the influence
of food and drink, temperature, emotions, etc.
Comparative. — In fishes, reptiles, and birds, uric acid re-
places urea, and is very abundant. In these animals most of
this substance is white. The urine is passed with the fasces.
In certain groups of invertebrates uric acid seems to be a
normal excretion.
THE SECRETION OF URINE.
By means of apparatus adapted to register the changes of
volume the kidney undergoes, it is found that this organ not
only responds to every general change in blood-pressure, but
to each heart-beat — that is, its volume varies momentarily.
This shows how sensitive it is to variations in blood-pressure.
Theories regarding the secretion of urine may be divided
into those that are almost wholly physical, partly physical,
and purely secretory: 1. To the first class belongs that of
Ludwig, which teaches that very dilute urine is separated from
the blood in the glomeruli, and by a process of osmosis and
absorption of water by the tubular capillaries is gradually
concentrated to the normal. 2. As an example of the second
class is that of Bowman, who maintained that the greater part
of the water and some of the more soluble and diffusible salts
422 COMPARATIVE PHYSIOLOGY.
are separated by the glomeruli but the characteristic constitu-
ents of the urine by the epithelium of the renal tubules. 3. As
an example of the third is the theory of Heidenhain, who attrib-
uted little to blood-pressure in itself, and much, if not the
whole, to the secreting activity of the epithelium of the tubules
more particularly. This physiologist showed that while liga-
BLOOD PRESSURE CURVE
VVWvVWA^
Fig. 319. — Blood-pressure curve and curve of the volume of the kidney; T, time-
curve, intervals indicate a quarter of a minute; A, abscissa (Stirling, after Koy).
ture of a vein raised the blood-pressure within a glomerulus, it
was not followed by any increase in the quantity of the secretion,
but by its actual arrest. He also showed that injection of a col-
ored substance (sodium sulphindigodate) into the blood, after the
pressure had been greatly lowered by section of the spinal cord,
led to its appearance in the urine ; and microscopic examination
showed that it had passed through the epithelial cells of the
tubules, not of the glomeruli.
It is found, however, that after the removal of a ligature
applied to the renal artery the urine is albuminous, showing
plainly that the cells have been injured by the operation ; hence
Heidenhain's experiment described above is not valid against
the blood-pressure theory. Moreover, too much must not be
inferred from the action of foreign substances under the ab-
normal conditions of such an experiment. While some physi-
ologists claim that the glomeruli are filtering mechanisms, they
explain that filtration is not to be understood in its ordinary
laboratory acceptation, but that the glomeruli discriminate as
to what they allow to pass, yet they in no way explain how
this is done. They make the whole process depend on blood-
pressure, and attribute little special action to the flat epithelium
of the Malpighian capsules.
Though we can not admit the full force of Heidenhain's ex-
periments as he interprets them, we still believe that his views
are most in harmony with the general laws of biology and the
EXCRETION BY THE KIDNEY. 423
special facts of renal secretion. More recently it lias been ren-
dered clear that physical theories of the work of the kidney can
not hold, even of the glomeruli, which are shown to be, as we
should have expected, true secreting organs. Now, there can
be no doubt that blood-pressure is a most important determin-
ing condition here as in other secreting processes, in the mam-
mal at all events ; but whether of itself or because of the influ-
ence it has on the rapidity of blood-flow, it is difficult to deter-
mine ; or rather whether solely to the latter, for that the con-
stant supply of fresh blood is a regular condition of normal
secretion there can be no doubt. Further, it seems probable that
blood-pressure has more to do with the secretion of water than
any other constituent of urine. But we maintain that it should
be called a genuine secretion, and that nothing is gained by
using the term " filtration " — on the contrary, that it is mislead-
ing, and tends to divert attention from the real though often
hidden nature of vital processes. The facts of disease and the
evidence of therapeutics, we think, all favor such a view of the
work of the kidneys.
Nerves having an influence over the secretion of urine simi-
lar to those acting on the digestive glands have not yet been
determined. The powerful influence of emotion, especially
well seen in the dog, over the secretion of urine shows that
there must be nervous channels through which the nerve-
centers act on the kidneys ; though whether the results are not
wholly dependent upon vaso-motor effects may be considered
as an open question by many. We think such a view im-
probable in the highest degree. The most recent investigations
would seem to show that the vaso-motor fibers run in the dor-
sal nerves, especially the eleventh, twelfth, and thirteenth, in
the dog, and that of these the vaso-constrictors are the best de-
veloped.
Pathological.— When the kidneys are excised, the ureters
ligatured, or when the former are so diseased as to be inca-
pable of performing their functions, death is the result, being
preceded by marked depression of the brain-centers, passing
into coma. Exactly which of the retained products brings
about these results is not known. They are likely due to sev-
eral, and it impresses on the mind the importance of those pro-
cesses by which the constantly accumulating waste is elimi-
nated.
424 COMPARATIVE PHYSIOLOGY.
THE EXPULSION OF URINE.
We now present in concise form certain facts on which to
base opinions as to the nature of the processes by which the
bladder is emptied.
It will be borne in mind that the secretion of urine is con-
stant, though of course very variable, that the urine is con-
veyed in minute quantities by rhythmically contractile tubes
(ureters) which open into the bladder obliquely ; and that the
bladder itself is highly muscular, the cells being arranged both
circularly and obliquely, with a special accumulation of the
circular fibers around the neck of the organ to form the sphinc-
ter vesicae.
1. It is found that the pressure which the sphincter of the
bladder can withstand in the dead is much less (about one
third) than in the living subject. 2. We are conscious of being
able to empty the bladder, whether it contains much or little
fluid. 3. We are equally conscious of an urgency to evacuation
of the vesical contents, according to the fullness of the organ,
the quality of the urine, and a variety of other conitions.
4. Emotions may either retard or render micturition urgent.
5. In a dog in which the cord has been divided in the dorsal
region some months previously, micturition may be induced
reflexly, as by sponging the anus. 6. In the paralyzed there
may be retention or dribbling of urine. 7. In cases of urethral
obstruction from a calculus, stricture, etc., there may be excess-
ive activity of the muscular tissue of the bladder walls. 8.
Evacuation of the bladder may occur in the absence of con-
sciousness (sleep).
The most obvious conclusions from these facts are that — 1.
The urine finds its way to the bladder partly through muscular
(peristaltic) contractions of the ureters, partly through gravity,
in man at all events, and partly from the pressure within the
tubules of the kidneys themselves. 2. The evacuation of urine
may take place independently of the will (see 8), and is a reflex
(5) act. 3. Micturition may be initiated by the will, which is
usually the case, when by the action of the abdominal muscles
a little urine is squeezed into the urethra, upon which afferent
impulses set up contractions of the bladder by acting on the
detrusor center of the cord and at the same time inhibit the
center presiding over the sphincter (if such there be), permit-
ting of its relaxation. 4. Emotions seem to interfere with the
EXCRETION BY THE KIDNEY.
425
Fig. &0.— Superior and general view of the genito-urinary apparatus in the stallion
with the axtenesr,(Chauveau). A, left kidney; B, right kiduev a b ureters- C C
suprarenal capsules; D, bladder; E. E. testicles; e, head of ep'ididimis; ',' tail
of epididimis; *, deferent canal; G, pelvic dilatation of deferent canal- H left
seminal vesicle; the right has been removed, along with the deferent canal of
426 COMPARATIVE PHYSIOLOGY.
same side, to show insertion of ureters into bladder; I, prostate; J, Cowper's
glands; K, membranous or intra-pelvic portion of urethral canal; L, its bulbous
portion; M, cavernous body of penis; in, m, its roots; N, head of penis; 1, ab-
dominal aorta; 2,2, arteries (renal) giving off principal capsular artery; 3, sper-
matic artery; 4, common origin of umbilical and arteries of bulb; 5, umbilical
artery; 6, its vesical branch; 7, internal artery of bulb; 8, its vesico-prostatic
branch.
ordinary control of the brain-centers over those in the spinal
cord. 5. It may be assumed that the normal tone of the
sphincter of the bladder is maintained reflexly by the spinal
cord. The unwonted muscular contraction associated with an
obstruction to the outflow of urine may be in part of nervous
origin, but is also, in all probability, owing in some degree to
the muscle-cells resuming an independent contractility, due to
what we recognize as the principle of reversion. The same is
seen in the heart, ureters, and similar structures.
Pathological. — There may be incontinence of urine from pa-
ralysis, the cerebral centers being unable to control those in
the spinal cord. Dribbling of urine may be due to retention in
the first instance, the tone of the sphincter being finally over-
come, owing to increase of pressure within the bladder. Over-
distention of the bladder may arise in consequence of lack of
tone in the muscular walls, though this is rare. Strangury is
due to excessive action of the walls of the bladder and the
sphincter, brought about reflexly, when the organ is unduly
irritable, as in inflammation, after the abuse of certain drugs
(cantharides), etc.
Comparative. — In man the last drops of urine are expelled
by the action of the bulbo-cavernosus muscle and perhaps some
others. In the dog and many other animals the regulated and
voluntary use of this muscle, marked in a high degree, produces
that interrupted flow so characteristic of the micturition of
these animals.
Summary. — Urine is in mammals a fluid of variable specific
gravity and reaction, yellow in color, and containing certain
salts, pigments, and nitrogenous bodies. The chief of the latter
is urea.
The kidneys and skin especially supplement one another,
and normally great activity of the one implies lessened ac-
activity of the other. This is availed of in the treatment of dis-
ease.
Both the Malpighian capsules and the renal tubules have a
true secretory function, though the larger pai^t of the water of
urine is secreted by the former. Blood-pressure is an important
EXCRETION BY THE KIDNEY. 427
condition of secretion, though it is likely that this is so chiefly
because it favors a rapid renewal of the blood circulating
through the organ. Whether there are nerves that influence
secretion directly, as in the case of the skin, is not determined.
Suppression of the renal functions leads to symptoms in
which the nervous system is recognized as suffering to the
extent often of coma, ending in death. The urine of most other
animals is more concentrated than that of man ; this secretion
in carnivora being acid, and in herbivora alkaline in reaction.
THE METABOLISM OF THE BODY.
In the widest sense the term metabolism may he conven-
iently applied to all the numerous changes of a chemical kind,
resulting from the activity of the protoplasm of any tissue or
oi*gan. In a more restricted meaning it is confined to changes
undergone hy the food from the time it enters till it leaves the
body, in so far as these are not the result of obvious mechanical
causes. The sense in which it is employed in the present
chapter will be plain from the context, though usually we shall
be concerned with those changes effected in the as yet compara-
tively unprepared products of digestion, by which they are ele-
vated to a higher rank and brought some steps nearer to the
final goal toward which they have been tending from the first.
As yet our attempts to trace out these steps have been little
better than the fruitless efforts of a lost traveler to find a road,
the general direction of which he knows, but the ways by which
it is reached only the subject of plausible conjecture. We
shall therefore not discuss the subject at length from this point
of view.
THE METABOLISM OF THE LIVER.
This organ has two well-recognized functions : 1. The for-
mation of bile. 2. The formation of glycogen. We have
already considered the first.
Glycogen may be obtained from the liver of mammals as a
whitish amorphous powder, having the chemical composition of
starch, and has in fact been termed animal starch.
By appropriate treatment it may be converted into sugar by
a process of hydration (CoHioOc + H2O = CoHiaOo).
The principal facts as to the storage of glycogen in the liver
may be briefly stated thus :
1. Glycogen has been found in the liver of a large number
THE METABOLISM OF THE BODY. 429
of groups of animals including some invertebrates. 2. Among
mammals it is most abundant wben the animal feeds largely
on carbohydrates. 3. It is found in the liver of the carnivora,
and in those of omnivora, when feeding exclusively on flesh.
4. When an animal starves (does not feed), the glycogen grad-
ually disappears. 5. A fat-diet does not give rise to glycogen.
6. During early foetal life glycogen is found in all the tissues,
but later it is restricted more and more to the liver, though
even in adixlts it is to be found in various tissues, especially the
muscles, from which it is almost never absent.
From the facts the inference is plain that glycogen is formed
from carbohydrate materials ; or, to be rather more cautious,
that the formation of this substance is dependent on the pres-
ence of such material in the food.
The Uses of Glycogen. — No positive statement can be made on
this subject. It is generally believed to be transformed into sugar.
What is the fate of the transformed glycogen ? What be-
comes of the sugar ? We can answer, negatively, that it is not
used up in the blood, it is not oxidized there ; but by what
tissues it is used or how it is made available in the economy is
a subject on which we are profoundly ignorant. The presence
of so much glycogen in the partially developed tissues of the
foetus points to its importance, and suggests its being a crude
material which is laid up to be further elaborated, as in vege-
tables, by the growing protoplasm.
METABOLISM OF THE SPLEEN.
The physiological significance of the peculiar structure of
this organ, though not yet fully understood, is much plainer
than it was till recently. The student is recommended to look
carefully into the histology of the spleen, especially the dis-
tribution of its muscular tissue and the peculiarities of its
blood-vascular system. It has already been pointed out that
there is little doubt that leucocytes are manufactured here even
in the adult, possibly also red cells ; and that the latter are dis-
integrated, and the resulting substances worked over, possibly
by this organ itself. This view is rendered probable, not only
by microscopic study of the organ, but by a chemical examina-
tion of the splenic pulp; for a ferruginous proteid, and numer-
ous pigments, of a character such as harmonizes with this con-
ception, are found,
430
COMPARATIVE PHYSIOLOGY.
The fact that the spleen-pulp does not agree in composition
with either blood or serum ; that it abounds in extractives such
Fig. 321. — Vertical section of a small superficial portion of human spleen, seen with
low power (Schafer). A, peritoneal and fibrous covering; b, trabecules; c, c, Mal-
pighian corpuscles, in one of which an artery is seen cut transversely, in the other,
longitudinally; d, injected arterial twigs; e, spleen-pulp.
as lactic, butyric, formic, and acetic acids, together with inosit,
xanthin, hypoxanthin, leuciu and uric acid — points to its being
Pio. 322.— Thin section of spleen-palp, highly magnified, showing mode of origin of
a small vein in the interstices of pulp (Scnafer). v, vein filled with blood-corpus-
cles, which are in continuity with others, hi. filling up interstices of retiform tissue
of pulp; w, wull of vein. The shaded bodies among red corpuscles are pale cor-
puscles.
THE METABOLISM OF THE BODY.
431
the seat of a complex metabolism, though neither the changes
themselves nor their purpose are well understood.
Nevertheless, it must be admitted that to recognize this was
a great advance upon the view that the spleen had no impor-
tant function, and that this was shown by the removal of
the organ without
change in the ani-
mal's economy.
But to believe
that there are no
such changes, and to
have clear proof of
it, are two different
things. As a matter
of fact, closer study
does show that in
some animals there
are alterations in the
lymphatic glands
and bone - marrow,
which organs are
undoubtedly manu-
facturers of blood-
cells.
These changes
Fig. 323. — Portion of spleen of cat, showing Malpighian
(lymphatic) corpuscle (after Cadiat). A, artery
around which corpuscle is placed; B, meshes of
spleen-pnip, injected; C, artery of corpuscle ramify-
ing in lymphatic tissue. The clear space around
corpuscle represents a lymphatic sinus.
are unquestionably compensatory, and that other similar ones
corresponding to comparatively unknown functions of the
spleen have not as yet been discovered is owing likely to our
failures rather than their real absence. We dwell for a mo-
ment on this, because it illustrates the danger of the sort of rea-
soning that has been applied in the case of this and other or-
gans; and it shows the importance of recognizing the force
of the general pi'inciples of biology, and also the desirability
of refraining from drawing conclusions that are too wide
for the premises. In every department of physiology it must
be more and more recognized that what is true of one group
of animals is not necessarily true of another, or even of other
individuals, though the differences in the latter case are of
course usually less marked. We have referred to this be-
fore, and shall do so again, for it is as yet but too little con-
sidered.
432 COMPARATIVE PHYSIOLOGY.
THE CONSTRUCTION OF FAT.
It is a well-known fact that, speaking generally, a diet rich
in carbohydrates favors fat formation, hoth in man and other
animals; though it is not to be forgotten that many persons
seem to be unable to digest such food, or, at all events, to as-
similate it so as to form fat to any great extent. Persons given
to excessive fat production are as frequently as not sparing
users of fat itself.
It is possible in man and probable in ruminants that fer-
mentations may occur in the intestines giving rise to fatty acids
which are possibly converted into fats by the cells of the villi
or elsewhere. Certain feeding experiments favor the view
that carbobydrates may be converted into fat or in some way
give rise to an increase in this substance ; for it is to be borne
in mind that fat may arise from a certain diet in various
ways other than its direct transformation into this substance
itself.
There are certain facts that make it clear that fat can be
formed from proteids: 1. A cow will produce more butter than
can be accounted for by the fat in her food alone. 2„ A bitch
which had been fed on meat produced more fat in her milk
than could have been derived directly from her food, and this,
when the animal was gaining in weight, which is usually to be
traced to the addition of fat ; so that the fat of the milk was
not, in all probability, derived from that of the dog's body ;
and, as will be seen presently, can be accounted for without
such a supposition. 3. It has been shown by analysis that 472
parts of fat were deposited in the body of a pig for every 100 in
its food.
These facts of themselves suffice to show that fat can be
formed from proteid, or at least that proteid food can of itself
give rise to a metabolism, resulting in fat formation; and the
latter is probably the better way to state the case in the present
condition of knowledge.
That fat is a real formation, dependent for its composition
on the work of living tissues, is clear from the well-known fact
that the fat of one animal differs from that of another, and that
it preserves its identity, no matter what the food may be, or. in
what form fat itself may be provided. Certain constituents of
the animal's fat may be wholly absent from the fat of its food,
yet they appear just the same in the fat produced under such
THE METABOLISM OF THE BODY. 433
diet. Even bees can construct their wax from proteicl, or use
unlike substances, as sealing-wax.
Pig. 324. — Section of mammary gland (udder and nipplel of cow (after Thanhoffcr)-
Ma, substance of gland; N nipple; A, acini of gland; m. d, milk-ducts; C, milk-
cisterns;/, folds in wide milk-ducts; S, section of sphincter muscle; s, external
skin; n. m. d, narrow milk-duct in nipple.
But histological examination of forming adipose tissue itself
throws much light upon the subject. Fat-cells are those in
which the protoplasm has been largely replaced by fat. The
28
434
COMPARATIVE PHYSIOLOG-Y.
latter is seen to arise in the former as very small globules
which run together more and more till they may wholly re-
place the original protoplasm.
The history of the mammary gland is, perhaps, still more
instructive. In this case, the appearance of the cells during
lactation and at other periods is entirely different. Fat may he
seen to arise within these cells and be extruded, perhaps in the
same way as an Amoeba gets rid of the waste of its food. So
far as the animal is concerned, milk is an excretion in a limited
sense.
It is, in the nature of the case, impossible to follow with
the eye the formation and separation of milk-sugar, casein, etc.
Fig. 325. — I. Acinus from mamma of a bitch when inactive (after Heidenhain). II.
During secretion of milk, a, b, milk-globules; c, d, e, colostrum-corpuscles; /,
pale cells.
But the whole process is plainly the work of the cells, and in
no mechanical sense a mere deposition of fat, etc., from the
blood ; and the same view applies to the construction of fat by
connective (adipose) tissue.
Whether fat, as such, or fatty acid, is dealt with without
being built up into the protoplasm of the cell, is not known ;
but, taking all the facts into the account, and considering the
behavior of cells generally, it seems most natural to regard the
construction of fat as a sort of secretion or excretion. To sup-
pose that a living cell acts upon material in the blood as a
workman in a factory on his raw material, or even as a chemist
does in the laboratory, seems to be too crude a conception of
vital processes. Until it can be rendered very much clearer
than at present, it is not safe to assume that their chemistry is
our chemistry, or their methods our methods. It maybe so;
but let us not, in our desire for simple explanations or undue
haste to get some sort of theory that apparently fits into our
THE METABOLISM OF THE BODY.
435
own knowledge, assume it gratuitously, in the absence of the
clearest proofs, especially when our failures on this supposition
are so numerous.
We may say, then, that fat is not merely selected from the
blood,' but formed in the animal tissues ; that fat formation
Fig. 326 —Microscopic appearances of— I. milk; II, cream; III, butter; IV, colostrum
of mare; V, colostrum of cow (after Thanhoffer).
may take place when the food consists largely of carbohydrates,
when it is chiefly proteid, or when proteid and fatty. In other
words, fat results from the metabolism of certain cells, which
is facilitated by the consumption of carbohydrate and fatty
food, but is possible when the food is chiefly nitrogenous. We
must, however, recognize differences both of the species and the
individual in this respect, as to the extent to which one kind of
food or the otber most favors fat formation (excretion). The
use of the adipose tissue as a packing to prevent undue escape
of heat is evident ; but more important purposes are probably
served, as will appear from later considerations.
Pathological.— Excessive fat formation, leading to the ham-
pering of respiration, the action of the muscles, and, to a certain
extent, many other functions of the body, does not arise in man
436 COMPARATIVE PHYSIOLOGY.
usually till after middle life, when the organism has seen its
best days. It seems to indicate, if we judge by the frequency
of fatty degeneration after disease, that the protoplasm stops
short of its best metabolism, and becomes degraded to a lower
rank ; for certainly adipose tissue does not occupy a high
place in the histological scale. Such pathological facts throw
a good deal of light upon the general nature of fat excre-
tion, as it would be better to term it, perhaps, and seem to
warrant the view that we have presented of the metabolic pro-
cesses.
Although the nerves governing the secretion of milk have
not been traced, there can be no doubt that the nervous system
controls this gland also. The influence of the emotions on both
the quantity and quality of the milk in the human subject and
in lower animals is well known.
Comparative. — While breeders recognize certain foods as
tending to fat formation and others to milk production, it is
interesting to note that their experience shows that race and
individuality, even on the male side, tell. The same conditions
being in all respects observed, one breed of cows gives more
and better milk than another, and the bull is himself able to
transmit this peculiarity ; for, when crossed with inferior breeds,
he improves the milking qualities of the latter. Individual
differences are also well known.
THE STUDY OF THE METABOLIC PROCESSES BY
OTHER METHODS.
It will be abundantly evident that our attempts to follow
the changes which the food undergoes from the time of its
introduction into the blood until it is removed in altered form
from the body has not been as yet attended with great success.
It is possible to establish relations between the ingesta and the
egesta, or the income and output which have a certain value.
It is important, however, to remember that, when quantitive
estimations have to be made, a small error in the data becomes
a large error in the final estimate ; one untrue assumption
may vitiate completely all the conclusions.
In discussing the subject we shall introduce a number of
tables, but it will be remembered that the results obtained by
one investigator differ from those obtained by another ; and
tbat in all of tbem there are some deviations from strict ac-
THE METABOLISM OP THE BODY. 437
curacy, so that the results must he regarded as only approxi-
mately correct. It is, however, we think, better to examine
such statistical tables of analyses, etc., than to rely on the mere
verbal statement of certain results, as it leaves more room for
individual judgment and the assimilation of such ideas as they
may suggest outside of the subject in hand.
The subject of diet is a very large one ; but it will be evi-
dent on reflection that, before an average diet can be prescribed
on any scientific grounds, the composition of the body and the
nature of those processes on which nutrition generally depends
must be known. . Not a little may be learned by an examina-
tion of the behavior of the body in the absence of all diet,
when it may be said to feed on itself, one tissue supplying
another. All starving animals are in the nature of the case
carnivorous.
For the cat an analysis has yielded the following :
Muscle and tendons. 45 "0 per cent.
Bones 147
Skin 12-0
Mesentery and adipose tissue 3*8
Liver 4*8
Blood (escaping at death) 6'0
Other organs and tissues 13'7
The large proportional weight of the muscles, the similarly
large amount of blood they receive, which is striking in the
case* of the liver, also suggest that the metabolism of these
structures is very active, and we should expect that they
would lose greatly during a starvation period. It is a matter of
common observation that animals do lose weight and grow
thin under such circumstances, which means that they must
lose in the muscles and the adipose tissue. Attempts have been
made to determine exactly the extent to which the various
tissues do suffer during complete abstinence from food, and
this may be gathered from the table given below.
It will not be forgotten that about three fourths of the body
is made up of water, so that the loss of a lai'ge amount of the
latter during starvation is to be expected.
In the case of a cat during a starvation period of thirteen
days 734 grammes of solids were lost, of which 248 grammes
were fat and 118 muscle — i. e., about one half of the total loss
was referable to these two tissues alone.
The other tissues lost as follows, estimated as dry solids :
438 COMPARATIVE PHYSIOLOGY.
Adipose tissues 97-0 per cent.
Spleen 63-l "
Liver 56-6
Muscles 30-2
Blood 17-6
Brain and spinal cord 0-0 "
It will be observed (a) that the loss of the fatty tissue was
greatest, nearly all disappearing ; (b) that the grandular struct-
ures were next in order the greatest sufferers ; (c) that after
them come the skeletal muscles.
Now, it has been already seen that these tissues all engage
in an active metabolism with the exception of adipose tissue.
The small loss on the part of the heart, which is still less for
the nervous system, is especially noteworthy. The loss of adi-
pose tissue is so striking that we must regard it as an especially
valuble storehouse of energy, available as required.
When we turn to the urine for information, it is found that
in the above case 27 grammes of nitrogen were excreted and
almost entirely, of course, in the form of urea; and since the
loss of nitrogen from the muscles amounted to 15 grammes, it
will appear that more than one half of the nitrogenous excreta
is traceable to the metabolism of muscular tissue. It has been
customary to account for the urea in two ways : first, as derived
from the metabolism of the tissues as stich, and continuously
throughout the whole starvation period ; and, secondly, from a
stored surplus of proteid which was assumed to be used up
rapidly during the early clays of the fasting, and was the luxus
consumption of certain investigators.
Comparative. — Experiment has shown that the length of
time during which different groups of animals can endure com-
plete withdrawal of food is very variable, and this applies to
individuals as well as species. That such differences hold for
the human subject is well illustrated by the history of the sur-
vivors of wrecks. Making great allowances for such devia-
tions from any such results as can be established by a limited
number of experiments, it may be stated that the human being
succumbs in from twenty-one to twenty-four days ; dogs in
good condition at the outset in from twenty-eight to thirty
days; small mammals and birds in nine days, and frogs in
nine months. Very much depends on whether water is allowed
or not — life lasting much longer in the former case. The very
young and the very old yield sooner than persons of middle
THE METABOLISM OF THE BODY. 439
age. It has been estimated that strong adults die when they
lose t4q of the body-weight. Well-fed animals lose weight more
rapidly at first than afterward.
Diet. — All experiments and observations tend to show that
an animal can not remain in health for any considerable period
without having in its food proteids, fats, carbohydrates, and
salts; indeed, sooner or later deprivation of any one of these
will result in death.
Estimates based on many observations have been made of
the proportion in which these substances should enter into a
normal diet.
For the herbivora from 1 to 8-9 (some claim 1 to 5|) is the
estimated ratio of nitrogenous to non-nitrogenous foods ; and 2
of the former to 1 of fat.
One conclusion that is obvious from analysis of foods is that,
in order to obtain the amount of proteids needed from certain
kinds, enormous quantities must be eaten and digested ; and as
there would be in such cases an excess of carbohydrates, fats,
etc., unnecessary work is entailed upon the organism in order
to dispose of this ; so that to feed a working horse entirely on
grass, a dog wholly on porridge, or a man on bread would be
very unwise.
FEEDING EXPERIMENTS (Ingesta and Egesta).
If all that enters the body in any form be known, and all
that leaves it be equally well known, conclusions may be drawn
in regard to the metabolism the food has undergone. The pos-
sible sources of fallacy will appear as we proceed.
The ingesta, in the widest sense, include the respired air as
well as the food ; though from the latter must be subtracted
the waste (undigested) matters that appear in the fasces. The
ingesta when analyzed include carbon, hydrogen, oxygen, ni-
trogen, sulphur, phosphorus, water, and salts, their source being
the atmosphere and the food-stuffs.
The egesta, the same, and chiefly in the form of carbonic an-
hydride, of water from the lungs, skin, alimentary canal, and
kidneys, of salts and water from the skin and kidneys, and of
nitrogen, chiefly as urea almost wholly from the kidneys. Usu-
ally in experimental determinations the total quantity of the
nitrogen of the urine is estimated. If free nitrogen plays any
part in the metabolic processes it is unknown.
440 COMPARATIVE PHYSIOLOGY.
A large number of feeding experiments have been made by
different investigators, chiefly, though. not exclusively, on the
lower animals. Some such method as the following has usu-
ally been pursued: 1. The food used is carefully weighed and a
sample of it analyzed, so that more exact data may be obtained.
2. The amount of oxygen used and carbonic anhydride exhaled,
as well as the amount of water given off in any form is esti-
mated. 3. The amount of the nitrogenous excreta is calculated,
chiefly from an analysis of the urine, though any loss by hair,
etc., is also to be taken into account.
It has been generally assumed that the nitrogen of the ex-
creta represents practically the whole of that element entering
the body. This has been denied by some investigators.
The respiratory products have been estimated in various
ways. One consists in measuring the quantity of oxygen sup-
plied to the chamber in which the animal under observation is
inclosed, and analyzing from time to time samples of the air as
it is drawn through the chamber; and on these results the total
estimates are based.
It will appear that even errors in calculating the composi-
tion of the food — and this is very variable in different samples,
e. g., of flesh; or any errors in the analysis of the urine, or in
the more difficult task of estimating the respiratory products,
may, when multiplying to get the totals, amount to serious de-
partures from accuracy in the end ; so that all conclusions in
such a complicated case must be drawn with the greatest cau
lion. But it can not be doubted that such investigations have
proved of much practical and some scientific value. The labor
they entail is enormous.
Nitrogenous Equilibrium.— It is possible to so feed an ani-
mal, say a dog, that the total nitrogen of the ingesta and egesta
shall be equal; and this may be accomplished without the ani-
mal losing or gaining weight appreciably or again while he is
gaining. If there be a gain, it can usually be traced to the
formation of fat, so that the proteid, we may suppose, has been
split up into a part that is constructed into fat and a part which
is represented by the urea, the fat being either used up or stored
in the body. Moreover, an analysis of a pig that had been fed
on a fixed diet, and a comparison made with one of the same
litter killed at the commencement of the experiment, showed
that of the dry nitrogenous food only about seven per cent in
this animal, and four per cent in the sheep had been laid away
THE METABOLISM OF THE BODY. 441
as dry proteid. It is perfectly plain, then, that proteid diet
does not involve only proteid construction within the body.
Comparative. — The amount of flesh which a dog, being a
carnivorous animal, can digest and use for the maintenance of
his metabolic processes is enormous; though it lias been learned
that ill-nourished dogs can not even at the outset of a feeding
experiment of this kind maintain the equilibrium of their body
weight on a purely flesh diet (fat being excluded). They at
once commence to lose weight — i. e., they draw upon their own
limited store of fat.
The digestion of herbivora being essentially adapted to a
vegetable diet, they can not live at all upon flesh, while a dog
can consume for a time without manifest harm ?15 to ^ of its
body-weight of this food.
Man, when fed exclusively on meat soon shows failure, he
being unable to digest enough to supply the needed cai'bohy-
drates, etc. But the large amount of urea in the urine of car-
nivorous animals generally, and the excess found in the urine
of man when feeding largely on a flesh diet, show that the pro-
teid metabolism is under such circumstances very active.
It is also a well-known observation that carnivorous ani-
mals (dogs) are more active and display to a greater extent
their latent ferocity, evidence of their descent from wild car-
nivorous progenitors, when like them they feed very largely on
flesh. The evidence seems to point pretty clearly to the con-
clusion that a nitrogenous (flesh) diet increases the activity of
the vital processs of the body, and especially the proteid me-
tabolism.
But in all these considerations it must be borne in mind that
the metabolic processes go on in the tissues and not in the
blood, and probably not in the lymph. Not that these fluids
(tissues) are without their own metabolic processes for and by
themselves; but what is meant to be conveyed is that the met-
abolic processes of the body generally do not take place in the
blood.
The Effects of Gelatin in the Diet.— Actual experiment shows
that this substance can not take the place of proteid, though it
also makes it evident that less of the latter suffices when mixed
with a certain proportion of gelatin. It will be borne in mind
that ordinary flesh contains, as we find it naturally in the car-
cass, not only some fat, but a good deal of fibrous tissue, which
can be converted by heating into gelatin.
442 COMPARATIVE PHYSIOLOGY.
Fats and Carbohydrates.— It is a matter of common observa-
tion and of more exact experiment tbat even a carnivorous ani-
mal thrives better on a diet of fat and lean meat than on lean
flesh alone. Thus, it has been found that nitrogenous equi-
librium was as readily established by a due mixture of fat and
lean as upon twice the quantity of lean flesh alone. It is plain,
then, that the metabolism is actually slowed by a fatty diet.
When an animal is given but little fat, none whatever is laid
up, but all the carbon of the fat can be accounted for in the
excreta, chiefly as carbonic anhydride. Again, the fatty por-
tion remaining constant, it has been found that increasing the
proteid leads not to a storage of the carbon of the proteid ex-
cess, but to an increased consumption of this element. It is
then possible to understand how excessive consumption of pro-
teids may lead, as seems to be the case, to the disappearance of
fat and loss of weight, so that a proteid diet increases not only
nitrogenous but non-nitrogenous metabolism. That carbohy-
drates mixed with a due proportion of the other constituents
of a diet do increase fat formation is well established ; though
there is no equally well-grounded explanation of how this is
accomplished. Upon the whole, it seems most likely that fat
can be directly formed from carbohydrates, or, at all events,
that they directly give rise to fat if they are not converted
themselves into that substance.
Comparative, — It is found that there are relations between
the food used and the quantity of carbonic dioxide expelled
which are instructive. The formula following show the amount
of oxygen necessary to convert a starch and a fat into carbonic
anhydride and water :
1. CoH1005 + 012=6(C02)+5(H20).
2. CeiH.cOe + O10u= 57(C02) + 52(H20).
It will be observed that in the first case the oxygen used to
oxidize the starch has all reappeared as C02, while in the sec-
ond only 114 parts out of 160 so reappear. As a matter of fact,
more of the oxygen used does in herbivora reappear as C02,
and less as water, while the reverse holds for the carnivora, the
proportion being, it is estimated, as ninety to sixty per cent.
This is to be explained by the character of the food in each
instance, for this relation no longer holds during fasting, when
the herbivorous animal becomes carnivorous in the sense that
it consumes its own tissues.
THE METABOLISM OF THE BODY. 443
The Effects of Salts, Water, etc., in the Diet.— When we
come to inquire as to the part salts play when introduced into
the blood, we soon find that our knowledge is very limited.
Sulphur, and especially phosphorus, seem to have some im-
portant use which quite eludes detection. It is important to
remember that certain salts are combined with proteids in the
body, possibly to a greater extent than we can learn from the
mere analysis of dead tissues.
Pathological. — The withdrawal of any of the important salts
of the body soon leads to disease, clear evidence in itself of their
great importance. This is notably the case in scurvy, in which
disease the blood seems to be so disordered and the nutrition
of the vessel- walls so altered that the former (even some of the
blood-cells) passes through the latter.
Water. — The use of water certainly has a great influence
over the metabolic processes of the body. The temporary ad-
dition or withdrawal of even a few ounces of water from the
regular supply of a dog in the course of a feeding experiment
greatly modfies the results obtained for the time. It is well
known that increase of water in the diet leads to a correspond-
ing increase in the amount of urea excreted. It is likely that
even yet we fail to appreciate fully the great part which water
plays in the animal economy.
THE ENERGY OF THE ANIMAL BODY.
As already explained, we distinguish between potential or
latent and actual energy. All the energy of the body is to be
traced to the influence of the tissues upon the food. Energy
may be estimated as mechanical work or as heat, and the one
may be converted into the other. All the processes of the
organism involve chemical changes, and a large proportion of
these are of the nature of oxidations ; so that speaking broadly,
the oxidations of the animal body are the sources of its energy ;
and in estimating the quantity of energy, either as heat or work,
that a given food-stuff will produce, one must consider whether
the oxidative processes are complete or partial ; thus, in the case
of proteid food, if we suppose that the urea excreted represents
the form in which the oxidative processes end or are arrested,
we must, in estimating the actual energy of the proteid, sub-
tract the amount of energy that would be produced were the
urea itself completely oxidized (burned.)
u±
COMPARATIVE PHYSIOLOGY.
If the amount of heat that a body will produce in its com-
bustion be known, then by the law of the conversion and equiv-
alence of energy the mechanical equivalent can be estimated in
that particular case.
The heat-producing power of different substances can be
directly learned by ascertaining the extent to which, when fully
burned (to water and carbonic anhydride), they elevate the
temperature to a given volume of water ; and this can at once be
translated into its mechanical equivalent of work, so that we
may say that one gramme of dry proteid would give rise to a
certain number of gramme-degrees of heat or kilogramme-
metres of work. A few figures will now show the relative
values of certain food -stuffs :
1 gramme proteid
1 gramme urea
Available energy of the proteid
Gram.-deg.
5,103
735
4,308
Kilomet.
2,161
311
1,850
The reason of the subtraction has been explained above.
Taking another diet in regard to whicb the estimates differ
somewhat from those given previously, but convenient now as
showing how equal weights of substances produce very dif-
ferent amounts of energy, we find that—.
100 grammes proteid yield
100 grammes fat yield . . .
240 grammes starch yield
Total
Gram.-deg.
430,800
900,900
938,880
2,281,580
Kilomet.
185,000
384,100
397,080
900,780
In other words nearly a million kilogramme-metres of en-
ergy are available from the above diet for one day, provided it
be all oxidized in the body.
Food-stuffs, then, with the oxygen of the air, are the body's
sources of energy. What are the forms in which its expendi-
ture appears % We may answer at once heat and mechanical
work ; for it is assumed that internal movements as those of
the viscera, and all the friction of the body, all its molcular
motion, all secretive processes, are to be regarded as finally
THE METABOLISM OF THE BODY. 445
augmenting the heat of the hody. Heat is lost by the skin,
lungs, urine, and faeces.
The division of foods into heat-producers and tissue-builders
is unjustifiable, as will appear from what has just been stated,
as well as from such facts as the production of fat from proteid
food, thus showing that the latter is indirectly a producer of
carbonic anhydride, assuming that fat is oxidized into that
substance.
ANIMAL HEAT.
Though a large part of the heat generated within the body
is traceable to oxidations taking place in the tissues, it is better
to speak of the heat as being the outcome of all the chemical
processes of the organism ; and though heat may be rendered
latent in certain organs for a time, in the end it must appear.
While all the tissues are heat-producers (thermogenic), the ex-
tent to which they are such would depend, we should suppose,
upon the degree to which they were the seat of metabolic pro-
cesses ; and actual tests establish this fact. Thus, among glands
the liver is the greatest heat-producer ; hence the blood from
this organ is the warmest of the whole body. The muscles also
are especially the thermogenic tissue.
The temperature of the blood in the hepatic vein is wanner
than that in the portal, a clear evidence that the metabolism of
this organ has elevated the temperature of the blood flowing
through it.
The temperature of the blood (its own metabolism being
slight) is a pretty fair indication of the resultant effect of the
production and the loss of heat.
For obvious reasons, the temperature of different parts of
the body of man and other animals varies.
The statements of observers in regard to the temperature of
various animals and of different parts of the body disagree in
a way that would be puzzling, were it not known how difficult
it is to procure perfectly accurate thermometers, not to mention
individual differences. The axillary temperature is in man
about 37° C. (98 6 F.); that of the mouth a little higher, and
of the rectum or vagina slightly more elevated. The mean
temperature of the blood is placed at 39° C. (102'2 F.).
Comparative. — The temperature of various groups of animals
has been stated to be as follows: Hen and pigeon, 42° (107'6 F.) ;
swallow, 4403° (111-25 F.) ; dolphin, 35'50 (95 "9 F.) ; mouse, 41-1°
446 COMPARATIVE PHYSIOLOGY.
(106 F.); snakes, 10° to 12° (50 to 53"6 F.) ; but higher in large
specimens (python). Cold-blooded animals have a tempera-
ture a little higher (less than 1° C. usually) than the surround-
ing air. During the swarming of bees the hive temperature
may rise from 32° to 40° (89 -6 to 104 F.). All cold-blooded
animals have probably a higher temperature in the breeding-
season. In our domestic mammals the normal temperature is
not widely different from that of man. In the horse the aver-
age is 37-5° to 38° (99 "5 to 100-4 F.) ; in the ass, 38° to 39-5° (100-4
to 103 F) ; in the ox, 38° to 38-5° (100*4 to 101 "3 F.) ; in the sheep
and pig, 39° to 40° (102-2 to 104 F.) ; in the cat, 38'5° to 39° (101*3
to 102-2 P.); in the dog, 38-5° (101-3 P.).
Variations in the average temperature are dependent on
numerous causes which may affect either the heat production
or heat loss : 1. Change of climate has a very slight but real
influence, the temperature being elevated a fraction of a degree
when an individual travels from the poles toward the equator,
and the same may be said of the effect of the temperature of a
warm summer day as compared with a cold winter one. The
wonder is that, considering the external temperature, the vari-
ation is so light. 2. Starvation lowers the temperature, and
the ingestion of food raises it slightly, the latter increasing, the
former decreasing, the rate of the metabolic processes. 3. Age
has its influence, the very young and the very old, in whom
metabolism (oxidation) is feeble, having a lower temperature.
This especially applies to the newly born, both among man-
kind and the lower mammals; and, as might be supposed, the
temperature falls during sleep, when all the vital activi-
ties are diminished. The same remark applies with greater
force to the hibernating state of animals. The temperature
of man does not vary more than about 1° C. during the twenty-
four hours.
It will be inferred, from the facts and figures already cited,
that different kinds of food have considerably different capacity
for heat production.
It is well known that an animal when working not only
feels warmer, but actually produces more heat.
It appears from a multitude of considerations that the body
is like a steam-engine, producing beat and doing work ; but it
is found that while a very good steam-engine, as a result of the
chemical processes going on within it, converts £ of the poten-
tial energy of its supplies into mechanical work, the other I
THE METABOLISM OP THE BODY. 447
appearing as heat, the animal hody produces } as work and | as
heat, from its income of food and oxygen.
While it is perfectly clear that it is in the metabolic pro-
cesses of the body that we must seek for the final cause of the
heat produced, it is incumbent on the physiologist to explain
the remarkable fact that the mammalian body maintains,
under a changing external temperature and other climatic
conditions, and with a varying diet, during rest and labor, a
temperature varying within, usually, no more than a fraction
of a degree centigrade. This we shall now endeavor to explain
in part.
The Regulation of Temperature.— It is manifest from the
facts adduced that so long as life lasts heat is being of necessity
constantly produced. If there were no provision for getting
rid of a portion of this heat, it is plain that the body would soon
be consumed as effectually as if it were placed in a furnace.
We observe, however, that heat is being constantly lost by the
breath, by perspiration (insensible), by conduction and radia-
tion from the surface of the body, and periodically by the
urine and faeces. We have seen that, while heat is being pro-
duced in all the tissues and organs of the body, some are es-
specially thermogenic, as the glands and muscles. The skin
presents an extensive surface, abundantly supplied with blood-
vessels, which when dilated may receive a large quantity of
blood, and when contracted may necessitate a much larger in-
ternal supply, in the splanchnic region especially. It is a mat-
ter of common observation that, when an individual exercises,
the skin becomes flushed, and so with the increased production
of heat, especially in the muscles (see page 195), there is a pro-
vision for unusual escape of the surplus ; at the same time
sweat breaks out visibly, or if not, the insensible perspiration
is generally increased ; and this accounts for an additional in-
crement of loss ; while the lungs do extra work and exhale an
increased quantity of aqueous vapor, so that in these various
ways the body is cooled. Manifestly there is some sort of rela-
tion between the processes of heat production and heat expendi-
ture. The vaso-motor, secretory, and respiratory functions are
modified. We may suppose that the various co-ordinations
effected, chiefly at all events through the central nervous sys-
tem, and not by the direct action of the heat upon local nerv-
ous mechanisms, or the tissues themselves directly, are re-
flexes.
448 COMPARATIVE PHYSIOLOGY.
The 'production of heat, however, seems to be equally under
the influence of the nervous system, though we know less about
the details of the matter.
A cold-blooded animal differs from a warm-blooded one in
that its temperature varies more with the surrounding medium:
hence the terms poikilothermer and homoiothermer for cold-
blooded and warm-blooded, would be appropriate.
Such an animal, as a frog or turtle, may have its chemical
processes slowed or quickened, almost like those going on in a
test-tube or crucible, by altering the temperature. Very differ-
ent is it, as we have seen, in the normal state of the animal with
any mammal. Hence hibernation or an allied state has be-
come a necessary protection for poikilothermers, otherwise they
would perish outright, and the groups become extinct in north-
ern latitudes.
It is plain that vaso-motor changes alone can not explain
these effects ; and, though possibly a part of the rise of tem-
perature, following exposure of the naked body in a cool air,
may be accounted for by the increased metabolism of internal
organs, accompanying the influx of blood caused by constric-
tion of the cutaneous capillaries, it is probable that in this as in
so many other instances the blood and circulation have been
credited with too much, and the direct influence of the nervous
system on nutrition and heat production overlooked or under-
estimated. The thermogenic center has not yet been definitely
located, though some recent investigations seem to favor a spot
in or near the corpus striatum for certain mammals. Some in-
vestigators also recognize a cortical heat-center. It has been
suggested that we may to advantage speak of a thermotaxic
(regulative of loss) and a thermogenic mechanism (regulative
of production), and even a thermolytic or discharging mechan-
ism. It has been further suggested that different nerve-fibers
may be concerned in the actual work of conveying the different
impulses of these respective mechanisms to the tissues ; and the
whole theory has been framed in accordance with the prevalent
conception of metabolism as consisting of anabolism and ca-
tabolism, or constructive and destructive processes. But these
theories have not yet been confirmed by experiments on ani-
mals, though they are, in the opinion of their authors, in har-
mony with the facts of fever. Certainly, any theory that will
imply that vital processes are more under the control of the
nervous system than has hitherto been taught, will, we think,
THE METABOLISM OF THE BODY. 449
advance physiology, as will shortly appear from our discussion
of the influence of the nervous system on the various metaholic
processes generally.
The phenomena observable in an animal gradually freezing
to death point strongly to the direct influence of the nervous
system on the production as well as the regulation of heat.
The circulation must of course be largely concerned, but it ap-
pears as though the nervous system refused to act when the
temperature falls below a certain point. A low temperature
favors hibernation, ' in which we believe the nervous system
plays the chief part, though the temperature in itself is not the
determining cause, as we have ourselves proved. The fact that
the whole metabolism of a hibernating animal is lowered, that
with this there is loss of consciousness much more profound
than in ordinary sleep, of itself seems to indicate that the nerv-
ous system is at the bottom of the whole matter.
Pathological, — It is found that many drugs and poisons
lower temperature, acting in a variety of ways. In certain dis-
eases, as cholera, the temperature may sink to 23° C. in extreme
cases before death supervenes. When the temperatui'e of the
blood is raised 6° to 8° C (as in sunstroke, etc.), death occurs ;
and it is well known that prolonged high tempei*ature leads to
fatty degeneration of the tissues generally. All the evidence
goes to show that in fever both the heat production and the
heat expenditure are interfered with ; or, at least, if not always,
that there may be in certain cases such a double disturbance.
In fever excessive consumption of oxygen and production of
carbon dioxide occur, the metabolism is quickened, hence its
wasting (consuming) effects ; the rapid respiration tends to in-
crease the thirst, from the extra amount of aqueous vapor ex-
haled. The body is actually warmest during the " cold stage "
of fever, when the vessels of the skin ai*e constricted and the
patient feels cold, because the internal metabolism is heightened ;
while the " sweating stage " is marked by a natural fall of tem-
perature. The fact that the skin may be dry and pale in fever
shows that the thermotaxic nervous mechanism is at fault; but
the chemical facts cited above (excess of CO2 etc.) indicate that
the thermogenic mechanism is also deranged.
20
450 COMPARATIVE PHYSIOLOGY.
SPECIAL CONSIDERATIONS.
If the student will now read afresh what has been written
under the above heading in relation to the subject of digestion,
it will probably appear in a new light. We endeavored to show
that, according to that general principle of correlation which
holds throughout the entire organism, and in harmony with
certain facts, we were bound to believe that digestion and as-
similation, or, to speak in other terms, the metabolic processes
of the various tissues, in a somewhat restricted sense, were
closely related. Beneath the common observation that " diges-
tion waits on appetite " lies the deeper truth that food is not
prepared in the alimentary canal (digested) without some rela-
tion to the needs of the system generally. In other words, the
voice of the tissues elsewhere is heard in the councils of the
digestive tract, and is regarded ; and this is effected chiefly
through the nervous system. Excess in eating may lead to
vomiting or diarrhoea — plain ways of getting rid of what can
not be digested.
Evolution. — We have already alluded to some of those modi-
fications in the form of the digestive organs that indicate an
unexpected plasticity, and impress the fact of the close rela-
tion of form and function. The conversion of a sea-gull into a
graminivorous bird, with a corresponding alteration in the na-
ture of the form of the stomach (it becoming a gizzard), with
doubtless modifications in the digestive processes, when re-
garded more closely, implies coadaptations of a very varied
kind. These are as yet but imperfectly known or understood,
and the subject is a wide and inviting field for the physiologist.
Darwin and others have indicated, though but imperfectly,
some of the changes that are to be regarded in animals as cor-
relations ; but in physiology the subject has received but little
attention as yet. We have in several parts of this work called
attention to it ; but the limits of space prevent us doing little
more than attempting to widen the student's field of vision by
introducing such considerations. The influence of climate on
metabolism, an undoubted fact, has many implications.
Any one who keeps a few wild animals in confinement un-
der close observation, and endeavors to ascertain how their
natural, self-chosen diet may be varied when confined, will
be astonished at the plasticity of their instincts, usually con-
sidered as so rigid in regard to feeding. These facts help one
THE METABOLISM OF THE BODY. 451
to understand how by the law of habit and heredity each group
of animals has come to prefer and flourish best upon a certain
diet. But habit itself implies an original deviation some time,
in which is involved, again, plasticity of nature and power to
adapt as well as to organize. Without this, evolution of func-
tion is incomprehensible ; but with this principle, and the
tendency for what has once been done to be easier of repetition,
and, finally, to become organized, a flood of light is thrown
upon the subject of diet, digestion, and metabolism generally.
On these principles it is possible to understand those race differ-
ences, even individual differences, which as facts must be patent
to all observers.
The principle of natural selection has clearly played a great
part in determining the diet of a species ; the surviving immi-
grants to a new district must be those that can adapt to the local
environment best, including the food which the region supplies.
The greater capability of resisting hunger and thirst in some
individuals of a species implies great differences in the meta-
bolic processes, though these are mostly unknown to us; and
the same remark applies to heat and cold
It seems clear that hibernation is an acquired habit of the
whole metabolism, with great changes in the functional condi-
tion of the nervous system recurring periodically, and, in fact,
dependent on these, by which certain large divisions, as the
reptiles, amphibians, and certain mammals among vertebrates,
are enabled to escape individual death and extinction as groups.
We may suppose that, for example, among invertebrates, by a
process of natural selection, those survived that could thus adapt
themselves to the environment ; while, among mammals, hiber-
nation may be considered as a process of reversion, perhaps, for
the homoiothermer becomes very much a poikilothermer during
hibernation, the latter reverting to a condition existing in lower
forms, and not wholly unlike that of plants in winter. This
can be understood on the principle of the origin of higher from
lower forms; otherwise it is difficult to understand why similar
states of the metabolism should prevail in groups widely sepa-
rated in form and function. If all higher groups bear a deriva-
tive relation to the lower, what is common in their nature, as
we usually find them, as well as the peculiar resemblances of
the metabolism of higher to lower forms in sleep, hibernation,
etc., can be understood in the light of physiological reversion.
The origin of a homoiothermic (warm-blooded) condition
452 COMPARATIVE PHYSIOLOGY.
itself is to be sought for in the principle of natural selection.
It was open to certain organisms, we may assume, either to
adapt to a temperature much below that of their blood, or to
hibernate; failing to make either adaptation would result in
death; and gradually, no doubt, involving the death of num-
berless individuals or species, the resisting power attained the
marvelous degree that we are constantly witnessing in all
homoiothermers.
The daily variations of the bodily temperature in homoio-
thermers is a beautiful example of the law of rhythm evident
in the metabolism. Hibernation is another such. While these
are clear cases, it is without doubt true that, did we but know
more of the subject, a host of examples of the operation of this
law might be instanced.
We can but touch on these subjects enough to show that
they deserve an attention not as yet bestowed on them ; and to
the thoughtful it will be evident that their influence on prac-
tical life might be made very great were they but rightly ap-
prehended.
THE INFLUENCE OF THE NERVOUS SYSTEM ON
METABOLISM (NUTRITION).
This subject is of the utmost importance, and has not re-
ceived the attention hitherto, in works on physiology, to which
we believe it is entitled, so that we must discuss it at some
length.
We may first mention a number of facts on which to base
conclusions : 1. Section of the nerves of bones is said to be fol-
lowed by a diminution of their constituents, indicating an
alteration in their metabolism. 2. Section of the nerves sup-
plying a cock's comb interferes with the growth of that ap-
pendage. 3. Section of the spermatic nerves is followed by de-
generation of the testicle. 4. After injury to a nerve or its
center in the brain or spinal cord, certain affections of the
skin may appear in regions corresponding to the distribution
of that nerve ; thus, herpes zoster is an eruption that follows
frequently the distribution of the intercostal nerve. 5. When
the motor cells of the anterior horn of the spinal cord or cer-
tain cells in the pons, medulla, or crus cerebri are disordered,
there is a form of muscular atrophy which has been termed
" active," inasmuch as the muscle does not waste merely, but
THE METABOLISM OE THE BODY. 453
the dwindling is accompanied by proliferation of the muscle
nuclei. 6. After neurotomy for navicular disease a form of de-
generation of the structures of the foot is not uncommon. 7.
After section of both vagi, death results after a period, varying
in time, as do also the symptoms with the animal. In some
animals pneumonia seems to account for death, since it is
found that, if this disease be prevented, life may, at all events,
be greatly prolonged. The pneumonia has been attributed to
paralyses of the muscles of the larynx, together with loss of
sensibility of the larynx, trachea, bronchi, and the lungs, so
that the glottis is not closed during deglutition, and the food,
finding its way into the lungs, has excited the disease by irrita-
tion. The possibility of vaso-motor changes is not to be over-
looked. In birds, death may be subsequent to pneumonia or
to inanition from paralysis of the oesophagus, food not being
swallowed. It is noticed that in these creatures there is fatty
(and sometimes other) degeneration of the heart, liver, stomach,
and muscles. 8. Section of the trigeminus nerve within the
skull has led to disease of the corresponding eye. This opera-
tion renders the whole eye insensible, so that the presence of
offending bodies is not recognized; and it has been both as-
serted and denied that protection of the eye from these pre-
vents the destructive inflammation. With the loss of sensi-
bility there is also vaso-motor paralysis, the intra-ocular ten-
sion is diminished, and the relations of the nutritive lymph to
the ocular tissues are altered. But all disturbances of the eye
in which there are vaso-motor alterations are not followed by
degenerative changes. 9. Degeneration of the salivary glands
follows suture of their nerves. 10. After suture of long-di-
vided nerves, indolent ulcers have been known to heal with
great rapidity. This last fact especially calls for explanation.
It will be observed, when one comes to examine nearly all such
instances as those referred to above, that they are complex.
Undoubtedly, in such a case as the trigeminus or the vagi,
many factors contribute to the destructive issue; but the fact
that many symptoms and lesions are concomitants does not, of
itself, negative the view that there may be lesions directly
dependent on the absence of the functional influence of nerve-
Abel's. We prefer, however, to discuss the subject on a broader
basis, and to found opinions on a wider survey of the facts of
physiology.
After a little time (a few hours), when the nerves of the sub-
45i COMPARATIVE PHYSIOLOGY.
maxillary gland have been divided, a flow of saliva begins and
is continuous till the secreting cells become altered in a way
visible by the microscope. Now, we have learned that proto-
plasm can discharge all its functions in the lowest forms oi
animals and in plants independently of nerves altogether.
What, then, is the explanation of this so-called u paralytic se-
cretion " of saliva ? The evidence that the various functions
of the body as a whole are discharged as individual acts or
series of acts correlated to other functions has been abundantly
shown; and, looking at the matter closely, it must seem un-
reasonable to suppose that this would be the case if there was
not a close supervision by the nervous system over even the
details of the processes. We should ask that the contrary be
proved, rather than that the burden of proof should rest on the
other side. Let us assume that such is the case ; that the entire
behavior of every cell of the body is directly or indirectly con-
trolled by the nervous system in the higher animals, especially
mammals, and ask, What facts, if any, are opposed to such a
view ? We must suppose that a secretory cell is one that has
been, in the course of evolution, specialized for this end. What-
ever may have been the case with protoplasm in its unspecialized
form, it has been shown that gland-cells can secrete independ-
ently of blood-supply (page 314, etc.) when the nerves going to
the gland are stimulated. Now, if these nerves have learned, in
the course of evolution, to secrete, then in order that they shall
remain natural (not degenerate) they must of necessity secrete;
which means that they must be the subject of a chain of meta-
bolic processes, of which the final link only is the expulsion of
formed products. Too much attention was at one time directed
to the latter. It was forgotten, or rather perhaps unknown,
that the so-called secretion was only the last of a long series of
acts of the cell. True, when the cells are left to themselves,
when no influences reach them from the stimulating nervous
centers, their metabolism does not at once cease. As we view
it, they revert to an original ancestral state, when they pei'-
formed their work, lived their peculiar individual life as less
specialized forms wholly or partially independent of a nervous
system. But such divorced cells fail; they do not produce
normal saliva, their molecular condition goes wrong at once,
and this is soon followed by departures visible by means of the
microscope. But just as secretion is usually accompanied by
excess of blood, so most functional conditions, if not all, de-
THE METABOLISM OF THE BODY. 455
mand an unusual supply of pabulum. This is, however, do
more a cause of the functional condition than food is a cause of
a man's working-. It may hamper, if not digested and assimi-
lated. It becomes, then, apparent that the essential for metab-
olism is a vital connection with the dominant nervous system.
It has been objected that the nervous system has a metab-
olism of its own independent of other regulative influences;
but in this objection it seems to be forgotten that the nervous
system is itself made up of parts which are related as higher
and lower, or at all events which intercommunicate and ener-
gize one another. We have learned that one muscle-cell has
power to rouse another to activity when an impulse has reached
it from a nervous center. Doubtless this phenomenon has
many parallels in the body, and explains how remotely a nerv-
ous center may exert its power. It enables one to understand to
some extent many of those wonderful co-ordinations (obscure
in detail) that are constantly taking place in the body. We
think the facts as they accumulate will more and more show,
as has been already urged, that the influence of blood-pressure
on the metabolic (nutritive) processes has been much over-
estimated. They are not essential but concomitant in the
highest animals. Turning to the case of muscle we find that
when a skeletal muscle is tetanized the essential chemical and
electrical phenomena are to be regarded as changes differing in
degree only from those of the so-called resting state. There is
more oxygen used, more carbonic anhydride excreted, etc. The
change in form seems to be the least important from a physio-
logical point of view. Now, while all this can go on in the
absence of blood or even of oxygen, it can not take place with-
out nerve influence or something simulating it. Cut the nerve
of a muscle, and it undergoes fatty degeneration, and atrophies.
True, this may be deferred, but not indefinitely, by the applica-
tion of electricity, acting somewhat like a nerve itself, and in-
ducing the approximately normal series of metabolic changes.
If, then, the condition when not in contraction (rest) differs
from the latter in all the essential metabolic changes in rate or
degree only ; and if the functional condition or accelerated
metabolism is dependent on nerve influence, it seems reason-
able to believe that in the resting condition the latter is not
withheld.
The recent investigations on the heart make such views as
we are urging' clearer still. It is known that section of the
456 COMPARATIVE PHYSIOLOGY.
vagi leads to degeneration of the cardiac structure. We now
know that this nerve contains fibers which have a diverse
action on the metabolism of the heart, and that, according
as the one or the other set is stimulated, so does the electri-
cal condition vary ; and everywhere, so far as known, a differ-
ence in electrical conditions seems to be associated with a
difference in metabolism, which may be one of degree only,
perhaps, in many instances — still a difference. The facts as
brought to light by experimental stimulation harmonize with
the facts of degeneration of the cardiac tissue on section of the
vagi ; but this is only clear on the view we are now presenting,
that the action of the nervous system is not only universal,
but that it is constant ; that function is not an isolated and
independent condition of an organ or tissue, but a part of a
long series of metabolic changes. It is true that one or more
of such changes may be arrested, just as all of them may go
on at a less rate, if this actual outpouring of pancreatic secre-
tion is not constant ; but secretion is not summed up in dis-
charge merely ; and, on the other hand, it would seem that in
some animals the granules of the digestive glands are being
renewed while they are being used up, in secreting cells. The
processes may be simultaneous or successive. Nor do we wish
to imply that the nervous system merely holds in check or in
a very general sense co-ordinates processes that go on unorigi-
nated "by it. We think the facts warrant the view that they are
in the highest mammals either directly (mostly) or indirectly
originated by it, that they would not take place in the absence
of this constant nervous influence. The facts of common ob-
servation, as well as the facts of disease, point in the strongest
way to such a conclusion. Every one has observed the in-
fluence, on not one but many functions of the animal, we might
say the entire metabolism, of depressing or exalting emotions.
The failure of appetite and loss of flesh under the influence of
grief or worry, tell a plain story. Such broad facts are of infi-
nitely more value in settling such a question as that now dis-
cussed than any single experiment. The best test of any theory
is the extent to which it will explain the whole round of facts.
Take another instance of the influence over metabolism of the
nervous system.
Every trainer of race-horses knows that he may overwork
his beast — i. e. , he may use his muscles so much as to disturb
the balance of his powers somewhere — very frequently his di-
THE METABOLISM OF THE BODY. 457
gestion ; but often there seems to be a general break — the whole
metabolism of the body seems to be out of gear ; and the same
applies to our domestic animals. If we assume a constant
nervcus influence over the metabolic processes, this is compre-
hensible. The centers can produce only so much of what we
may call nervous force, using the term in the sense of directive
power; and if this be unduly diverted to the muscles, other
parts must suffer.
On this view also the value of rest or change of work
becomes clear. The neiwous centers are not without some re-
semblance to a battery ; at most, the latter can generate only a
definite quantity of electricity, and, if a portion of this be di-
verted along one conductor, less must remain to pass by any
other.
It is of practical importance to recognize that under great
excitement unusual discharges from a nerve-center may lead
to unwonted functional activity; thus, under the stimulus of
the occasion an animal may in a race originate muscular con-
tractions that he could not call forth under other circumstances.
Such are always dangerous. We might speak of a reserve or
residualnerve force, the expenditure of which results iu serious
disability.
It seems that the usually taught views of secretion and
nutrition have been partial rather than erroneous in themselves,
and it is a question whether it would not be well to substitute
some other terms for them, or at least to recognize them more
clearly as phases of a universal metabolism. We appear to be
warranted in making a wider generalization. To regard pro-
cesses concerned in building up a tissue as apart from those that
are recognized as constituting its function, seems with the knowl-
edge we at present possess, to be illogical and unwise. Whether,
in the course of evolution, certain nerves, or, as seems more
likely, certain nerve-fibers in the body of nerve-trunks, have
become the medium of impulses that are restricted to regulat-
ing certain phases of metabolism — as e. g., expulsion of formed
products in gland-cells — is not, from a general point of view,
improbable, and is a fitting subject for fui'ther investigation.
But it will be seen that we should regard all nerves as " tro-
phic " in the wider sense. What is most needed, apparently, is
a more just estimation of the relative parts played by blood
and blood- pressure, and the direct influence of the nervous
system on the life-work of the cell.
458 COMPARATIVE PHYSIOLOGY.
We must regard the nervous centers as the source of cease-
less impulses that operate upon all parts, originating and con-
trolling the entire metabolism, of which what we term func-
tions are but certain phases, parts of a whole, but essential for
the health or normal condition of the tissues. Against such a
view we know no facts, either of the healthy or disordered or-
ganism.
Summary of Metabolism.— Very briefly and somewhat in-
completely, we may sum up the chief results of our present
knowledge (and ignorance) as follows:
Glycogen is found in the livers of all vertebrate and some
invertebrate animals. The quantity varies with the diet, being
greatest with an excess of carbohydrates.
Glycogen may be regarded as stored material to be convert-
ed into sugar, as required by the organism ; though the exact
use of the sugar and the method of its disposal are unknown.
Fat is not stored up in the body as the result of being
merely picked out from the blood ready made ; but is a genuine
product of the metabolism of the tissues, and may be formed
from fatty, carbohydrate, or proteid food. This becomes es-
pecially clear when the difference in the fat of animals from
that on which they feed is considered, as well as the direct re-
sults of feeding experiments, and the nature of the secretion of
milk.
The liver seems to be engaged in a very varied round of meta-
bolic processes ; the manufacture of bile, of glycogen, of urea,
and probably of many other substances, some known and
others unknown, as chemical individuals. Urea is in great
part probably only appropriated by tbe kidney-cells (Amoeba-
like) from the blood in which it is found ready made ; though
it may be that a part is formed in these cells, either from
bodies some steps on the way toward urea, or out of their pro-
toplasm, as fat seems to be by the cells of the mammary gland.
The leucin (and tyrosin ?) of the digestive canal sustains
some relation to the manufacture of urea by the liver, and pos-
sibly by tbe spleen and other organs ; for a proteid diet increases
these products, and also the urea excreted. Creatin, one of the
products of proteid metabolism, and possibly allied bodies, may
be considered as in a certain sense antecedents of urea ; uric-acid,
however, does not seem to be such, nor is it to be regarded as a
body that has some of it escaped complete oxidation, but rather
as a result of a distinct departure of the metabolism ; and there
THE METABOLISM OF THE BODY. 459
are facts which seem to indicate that the uric-acid metabolism
is the older, from an evolutionary point of view, and that in
mammals, and especially in man, as the results of certain errors
there may be a physiological (or pathological) reversion. Hip-
puric acid, as replacing uric acid in the herbivora, maybe re-
garded in a similar light.
Our knowledge of the metabolism of the spleen, beyond its
relations to the formation of blood-cells and their disintegra-
tion, is in the suggestive rather than the positive stage. It
seems highly probable that this organ plays a very important
part, the exact nature of which is as yet unknown.
When an animal starves, it may be considered as feeding on
its own tissues, the more active and important utilizing the
others. Notwithstanding, organs with a very active metabo-
lism, as the muscles and glands, lose weight to a large extent.
The presence of urea to an amount not very greatly below the
average in health, shows that there is an active proteid metabo-
lism then as at all times in progress.
General experience and exact experiments prove that, while
an animal's diet may be supplied with special regard to fatten-
ing, to increase working power, or simply to maintain it in
health, as evidenced by breeding capacity, form, etc., in all
cases there must be at least a certain minimum quantity of each
of the food-stuffs. No one food can be said to be exclusively
fattening, heat-forming, or muscle-forming.
A carbohydrate diet tends to production of fat ; proteid food
to supply muscular energy, but the latter also produces fat, and
a diet of proteid mixed with fat or gelatin will serve the pur-
poses of the economy better than one containing a very much
larger quantity of proteid alone. Muscular energy, as is to be
inferred from the excreta, is not the result of nitrogenous me-
tabolism alone; and in arranging any diet for man or beast the
race and the individual must be considered. Animals can not
be treated as machines, like engines using similar quantities of
fuel ; though this holds far more of man than the lower ani-
mals— i.e., the results may be predicted from the diet with far
more certainty in their case than for man.
Food is related to excreta in a definite way, so that all that
enters as food must sooner or later appear as urea, salts, car-
bonic anhydride, water, etc. These are individually to be re-
garded as the final links in a long chain of metabolic processes,
or rather a series of these. Fats and carbohydrates are repre-
460 COMPARATIVE PHYSIOLOGY.
sented finally as carbonic anhydride and water principally,
proteids as urea.
Nitrogenous foods may be regarded as accelerating the
metabolic processes generally and proteid metabolism in par-
ticular, while fats have the reverse effect ; hence fat in the diet
renders a less quantity of proteid sufficient. Gelatin seems to
act when mixed with proteid food either like an additional
quantity of proteid, or possibly like fat, at all events under such
circumstances less proteid suffices.
These facts have a bearing not only on health but on econ-
omy, in the expenditure for food.
Salts hold a very important place in every diet, though
their exact influence is in great part unknown. The heat of
the body is the resultant of all 'the metabolic processes of the
organism, especially the oxidative ones. Certain food-stuffs
have greater potential capacity for heat formation than others ;
but, finally, the result depends on whether the organism can
best utilize one or the other.
A certain body temperature, varying only within narrow
limits, is maintained, partly by regulation of the supply and
partly by the regulation of the loss.
Both these are, in health, under the direction of the nervous
system, and both are co-ordinated by the same. Loss is chiefly
through the skin and lungs ; gain chiefly through the organs
of most active metabolism, as the muscles and glands.
Vaso-motor effects play a great part in the escape of heat.
Animals may be divided into poikilothermers and homoio-
thermers, or cold-blooded and warm-blooded animals, accord-
ing as their body heat varies with or is independent of the ex-
ternal changes of temperature. All the facts go to show that
in mammals the processes of the body (metabolism) can con-
tinue only within a slight range of variations in temperature,
though the upward limit is narrower than the downward.
Upon the whole, the evidence justifies the conclusion that
the nervous system is concerned in all the metabolic processes
of the body in mammals including man, and that, as we descend
the scale, the dominion of the nervous system becomes less till
we reach a point when protoplasm goes through the whole
cycle of its changes by virtue of its own properties uninfluenced
by any modification of itself in the form of a nervous system.
THE SPINAL CORD.— GENERAL.
Among the higher vertebrates the spinal cord is found to
consist of nerve-cells, nerve-fibers, and a delicate connective tis-
sue binding them together ; while these different structures are
arranged in definite forms, so that a cross-section anywhere pre-
sents a characteristic appearance, the more important gangli-
onic nerve-cells being internal and forming a large part of
the gray matter of the cord. All the various regions of this
organ or series of organs are connected with one another,
white with white and gray matter, as well as white with gray
substance.
While we do not attempt to furnish a complete and detailed
account of the anatomy of the cord or other parts of the nervous
system, for which the student is referred to works on anatomy,
we would remind him that the spinal cord is situated within- a
bony case with joints permitting of a certain amount of move-
ment, variable in different regions. Inasmuch as the cord itself
does not fill its bony covering, but floats in fluid and tethered
to the walls by bands of connective tissue, it is well protected
from laceration, bruising, or concussion. Like the brain, it has
a protective tough outer membrane (dura mater) with a closer-
fitting iuner covering abounding in blood-vessels (pia mater).
The white matter of the cord invests the horns of gray
matter and is made up of nerve-fibers wanting the outer sheath.
Here, as elsewhere, these fibers have only a conducting func-
tion ; they do not originate nervous impulses. The gray matter,
on the other hand, abounds in cells, some of them with many
processes, that can originate, modify, and conduct impulses.
Certain well-recognized groups of these cells are arranged in
columns throughout the cord, as shown in the accompany-
ing figures. The supporting basis for these cells (neuroglia) is
the most delicate form of connective tissue known.
The cord may be regarded either as an instrument for the
Fio. 32'
Fio. 328.
THE SPINAL CORD.— GENERAL.
463
Fig. 327.— General view of spinal cord (Chauveau). A, cervical bulb; B, lumbar bulb;
C, cauda equina.
Fig. 328.— Segment of spinal cord at the cervical bulb, or brachial plexus, showing its
upper face and the roots of the spinal nerves (Chauveau). A, superior roots; J5,
inferior roots; C, multiple ganglia of superior roots; D, single ganglion on an
exceptional pair; E, £, upper roots passing through the envelopes.
reception and generation of impulses independent of the brain ;
or as a conductor of afferent and efferent impulses destined for
the brain or originating in that organ. As a matter of fact,
however, it is better to bear in mind that the cord and brain
constitute one organ or chain of organs, which, as we have
learned from our studies in development, are differentiations
of one common track, originating from the epiblast.
While the brain and the cord may act independently to a
Fig. 329.— Transverse section of spinal cord of child six months old, at middle of lum-
bar region, showing especially the fibers of gray substance. 1 x 20. (After Ger-
lach.) a, anterior columns; b. posterior columns; c, lateral columns: <l. anterior
roots; e, posterior roots; f, anterior white commissure; r/, central canal lined by
epithelial cells; h, connective-tissue substance surrounding it; i. transverse fibers
of gray commissure in front, and k, the same behind central canal: /. I wo veins
cut across: m. anterior eornua: it, great lateral cell group of anterior cornua; o,
lesser anterior cell group (column): px smallest median cell group; q, posterior
cornua;n ascending fasciculi in posterior cornua; *', substantia gelatinosa.
464
COMPARATIVE PHYSIOLOGY.
Fig. 330.— Group of cells in connection with anterior roots of spinal nerves, as seen in
transverse section of spinal cord of sheep (after Flint and Dean). A, emergence
of anterior roots from gray matter; b, b, b, cells connected both with each other
and with fibers of anterior roots.
very large extent, as may be shown by experiment, yet it can
not be too well borne in mind that in the actual normal life of
an animal such purely independent behavior must be exceed-
ingly rare. We are constantly in danger, in studying a sub-
ject, of making in our minds isolations which do not exist in
nature. When one accidentally sits upon a sharp object, he
THE SPINAL CORD.— GENERAL.
465
Fig. 331.— Division of a slender nerve-fiber, and communication of its branches with
highly ramifying processes of two nerve-cells from spinal cord of ox. 1 x 150.
(After Gerlach.)
rises suddenly without a special effort of will power; he expe-
riences pain, and has certain thoughts about the object, etc.
30
466
COMPARATIVE PHYSIOLOGY.
Now, in reality this is
very complex, though it
can be analyzed into its
factors. Thus, afferent
nerves are concerned, the
spinal cord as a reflex
center, efferent nerves to
the muscles called into
action, the cord as a con-
ductor of impulses which
result in sensations, emo-
tions, and thoughts refer-
able to the brain ; so that
if we would grasp the state
of affairs it is of impor-
tance to so combine the
various processes in our
mental conception that it
shall in our minds form
that whole which corre-
sponds with nature, as we
have been insisting upon
in the last chapter. With
this admonition, and as-
suming a good knowledge
of the general and minute
Fig. 332.— Multipolar ganglion cell from anterior , » ,-, ■,
gray matter of spinal cord of ox (after Dei- anatomy OI Uie spinal
ters). a, axis cylinder process; b, branched cor(] we shall nrc-ceed to
processes. - "
discuss its functions.
THE REFLEX FUNCTIONS OF THE SPINAL CORD.
The following experimental observations may readily be^
made by the student himself: Let a decapitated frog be sus-
pended freely (from the lower jaw). It hangs motionless and
limp at first, but when it recovers from the shock (abolition of
function) to the spinal cord produced by the operation, it may
be shown that this organ is functional: 1. When a piece of
bibulous paper dipped in dilute acid is placed upon the thigh,
the leg is drawn up and wipes away the offending body. 2. If
the paper be placed on the anus, both legs may be drawn up,
either successively or simultaneously. 3. If the leg of one
THE SPINAL CORD.— GENERAL. 467
side be allowed to hang in the dilute acid, it will he withdrawn.
4. If a small piece of blotting-paper dipped in the acid be
placed on the thigh, and the leg of that side gently held, the
other may be drawn up and remove the object.
It may be noticed that in every case a certain interval of
time elapses before the result follows. Upon increasing the
strength of the acid very much this interval is shortened, and
the number of groups of muscles called into action is increased.
Again, the result is not the same in all respects when the nerve
of the leg is directly stimulated, as when the skin first receives
the impression. Section of the nerves of the parts abolishes
these effects; so also does destruction of the spinal cord, or the
part of it with which the nerves of the localities stimulated are
connected ; and more exact experiments show that in the ab-
sence of the gray matter the section of the posterior or anterior
roots of the nerves also renders such manifestations as we have
been describing impossible.
These experiments and others seem to show that an afferent
nerve, an efferent nerve, and one or more central cells are
necessary for a reflex action ; that the latter is only a perfectly
co-ordinated one when the skin (end-organs) and not the nerve-
trunks are stimulated ; that there is a latent period of stimula-
tion, suggesting a central " summation " of impulses necessary
for the effect ; that the reflex is not due to the mere passage of
impulses from an afferent to an efferent nerve through the
cord, but implies important processes in the central cells them-
selves. The latter is made further evident from the fact that
(1) strychnia greatly alters reflex action by shortening the
latent period and extending the range of muscular action, which,
it has been shown, is not due to changes in the nerves them-
selves. A very slight stimulus suffices in this instance to cause
the whole body of a decapitated frog to pass into a tetanic
spasm. We must suppose that the processes usually confined
to certain groups of central cells have in such a case involved
others, or that the " resistance " of the centers of the cord has
been diminished, so that many more cells are now involved;
hence many more muscles called into action. Normally there is
resistance to the passage of an impulse to the opposite side of the
cord, as is shown by the fact that when a slight stimulus is ap-
plied to the leg of one side the reflex is confined to this member.
It is evident, then, that the reflex resulting is dependent on
(1) the location of the stimulus, (2) its intensity and duration.
46 S
COMPARATIVE PHYSIOLOGY.
(3) its character, and (4) the condition of the spinal cord at the
time. Occasionally on irritating one fore-limb the opposite
hind one answers reflexly. Such is a "' crossed reflex," and is
the more readily induced in animals the natural gait of which
involves the use of one fore-leg and the opposite hind-limb
together.
MOTOR
BRANCH
SENSORY?
bbucs
Fig. 333.— Diagram of a spinal segment showing component parts (Ranney).
Fig. 334.— Diagrammatic representation to illustrate the reflex arc (Bramwell and Ran-
ney). 1, 2, sensory fibers; 3, motor-cell of anterior horn; 4, motor-fiber connected
with 3 and passing out by anterior root to muscle; 5, fiber joining ganglionic cell
(3) with crossed pyramidal tract, C. P. C; G, ganglion on root of posterior spinal
nerve; 7, fiber joining 3 with Torek's column, T. Fiber 2 is represented as pass-
ing through Burdach's column to reach the cell, 3.
Reflexes are often spoken of as purposive, and suggest at
first intelligence in the cord; but such phenomena are explained
readily enough without such a strained assumption.
Evolution, heredity, and the law of habit, apply here as else-
where. The relations of an animal to its environment must
necessarily call into play certain nervo-muscular mechanisms,
THE SPINAL CORD.— GENERAL. 469
which from the law of habit come to act together when a
stimulus is applied. Naturally those that make for the welfare
of the animal are such as are most used under the influence of
the intelligence of the animal — i. e., of the domination of the
higher cerebral centers, so that when the latter are removed it
is but natural that the old mechanisms should be still employed.
Moreover, the reflex movements are not always beneficial, as
when a decapitated snake coils itself around a heated iron
under reflex influence, which is readily enough understood if
we remember the habit of coiling around objects, and what
this involves— viz., organized tendencies.
Inhibition of Reflexes. — It can be shown in the case of a frog
that still retains its optic lobes and the parts of the brain pos-
terior to them that, when these are stimulated at the same time
as the leg, the reflex, if it occurs at all, is greatly delayed.
On the other hand, in the case of dogs, from which a part
of the cerebral cortex has been removed, the reflexes are much
more prominent than before. Experience teaches us that the
acts of defecation, micturition, erection of the penis, and many
others, are susceptible of arrest or may be prevented entirely
when the usual stimuli are still active, by emotions, etc.
These and numerous other facts tend to show that the higher
centers of the brain can control the lower; and it is not to be
doubted that pure reflexes during the waking hours of the
higher animals, and especially of man, are much less numerous
than among the lower vertebrates. The cord is the servant of
the brain, and a faithful and obedient one, except in cases of
disease, to some forms of which we have already referred.
THE SPINAL CORD AS A CONDUCTOR OF IMPULSES.
It is to be carefully borne in mind now, and when studying
the brain, that a conducting path in the nervous centers is not
synonymous with conducting fibers. The cells themselves
and the neuroglia probably are also conductors. We shall
now endeavor to map out, as established by the method of
Flechsig, Waller, and others, the main fiber tracts of the spinal
cord.
1. Antero-median Columns (columns of Turck). — These
probably decussate in the cervical region, where they are most
marked, constituting the direct or uncrossed pyramidal tract
and disappear in the lower dorsal region.
470
COMPARATIVE PHYSIOLOGY.
PR<
Secondary degeneration ensues in these tracts upon certain
brain lesions, in the motor regions.
2. Crossed Pyramidal Tracts. — They pass forward to form
part of the anterior pyramids of the medulla after decussation
in their lower part. Simi-
larly to the first, degenera-
tion follows in these tracts
when there are brain - le-
sions of the motor area.
Hence, both of these consti-
tute descendingmotor paths.
3. Anterior Fascicidi
(fundamental or ground
bundle). — They possibly
connect the gray matter of
the cord with that of the
medulla.
4. Anterior Radicular
Zones, in the anterior part
of the lateral column.
5. Mixed Lateral Col-
umns.— These and the pre-
Fig. 335.— Diagrammatic representation of col- _ .
umns and conducting paths in spinal cord ceding are functionally sim-
in upper dorsal region (after Flint and
Landois). AR. AR, anterior roots of spi-
nal nerves; PR, PR, posterior roots; A,
columns of Ttirck (antero- median col-
umns) ; B, anterior fundamental fascicu-
lus; C, columns of Goll; D. columns of
Burdach; E, E, anterior radicular zones;
F, F, mixed lateral columns; G, G, crossed
pyramidal tracts; II, II, direct cerebellar trophic cells both above and
fibers. t t
below.
6. Direct Cerebellar Tracts. — These bundles, passing by the
funiculi graciles or posterior pyramids of the medulla, reach
the cerebellum by its inferior peduncles.
These fasciculi enlarge from their site of origin in the lum-
bar cord upward. After section of the cord they show ascend-
ing degeneration, so that it seems probable that their trophic
cells are to be referred to the posterior gray cornua of the cord,
which they connect in all probability with the cerebellum.
7. Columns of Burdach (postero-lateral columns). — This
tract is connected with the restiform bodies and reaches the
cerebellum by the inferior peduncles. Secondary degenera-
tions do not occur in these fasciculi, so that it seems likely that
they connect nerve-cells at different levels in the cord; and
ilar to 3. Neither 3, 4, nor
5 degenerate, on section of
the cord, from which it is
inferred that they have
THE SPINAL CORD.— GENERAL. 471
they may also connect the posterior gray cornua with the cere-
bellum as 6.
Columns of Goll (postero-median columns). — They do not
extend beyond the lower dorsal or upper lumber region ; and
their fibers pass to the funiculi graciles of the medulla. Ascend-
ing degeneration follows section of these columns.
The degenerations referred to above are visible by the micro-
scope, and of the character following section of nerves. It is
probable that they are the later stages of a primary molecular
derangement in consequence of interference with that continu-
ous functional connection between all parts on which what has
been called nutrition, but which we have shown is but a phase
of a complex metabolism, depends.
Decussation. — Sections of the cord, when confined to one lat-
teral half, are followed by paralysis on the same side and loss of
sensation, confined chiefly to the opposite half of the body be-
low the point of section. The results of experiment, patho-
logical investigation, etc., have rendered it clear that — 1. The
great majority of the fibers passing between the periphery and
the brain decussate somewhere in the centers. 2. Afferent fibers
cross almost directly but also to some extent along the whole
length of the cord from then' point of entrance, the decussation
being, however, completed before the medulla is passed. 3.
Motor or efferent fibers decussate chiefly in the medulla, though
crossing is continued some distance down the cord, such latter
fibers being but a small portion of the whole. This fact is best
established, perhaps, by noting the results of brain-lesions.
With few exceptions, susceptible of explanation, a lesion of one
side of the cerebrum is followed by loss of motion of the oppo-
site side of the body. These are all central, well-established
truths. It is also now pretty well determined that voluntary
motor impulses descend by the pyramidal tracts, both the direct
and the crossed. That the posterior columns of the cord are in
some way concerned with sensory impulses there is no doubt ;
but when an attempt is made to decide details, great difficulties
are encountered. Experiments on animals are of necessity very
unsatisfactory in such a case, from the difficulty experienced in
ascertaining their sensations at any time, and especially when
disordered.
Pathological. — A good deal of stress has been laid upon
the teachings of locomotor ataxia in the human subject. The
symptoms of this disease are found associated with lesions of
472
COMPARATIVE PHYSIOLOGY.
the posterior columns of the cord. The essential feature is an
inability to co-ordinate movements, though muscular power
may he unimpaired. But such inco-ordination is not usually
the only symptom ; and, while the disease seems usually to
begin in Burdach's columns, the columns of Goll, the posterior
nerve-roots, and even the cells of the posterior cornua, may be
involved, so that the subject becomes very complicated. Co-
ordination of muscular movements is normally dependent upon
certain afferent sensory impulses, themselves very complex. It
is to be remembered also that there are numberless connecting
links between the two sides of the cord and between its different
columns of an anatomical kind, not to mention the possibly
numerous physiological (functional) ones.
Fig. 336. — Diagram to illustrate probable course taken by fibers of nerve-roots on en-
tering spinal cord (Schafer).
We have stated above that section of one lateral half of the
cord is followed by loss of sensation on the opposite side of the
body ; but directly the contrary has been maintained by other
observers; while still others contend that the effects are not
confined to one side, though most pronounced on the side of
the section. The same remark applies to motion.
While there is considerable agreement as to the pyramidal
tracts of the lateral column, the functions of the rest of these
THE SPINAL CORD.— GENERAL.
473
divisions of the cord are by no means well established. It is
possible that vaso-motor, respiratory, and probably other kinds
of impulses, pass by portions of the lateral tracts other than
the crossed pyramidal. When a lateral half of the cord is
divided, the loss of function is not permanent in all instances,
but has been recovered from without any regeneration of the
divided fibers; and even when a section has been made higher
up on the opposite side, partial recovery has again followed ;
so that it would appear that impulses had pursued a zigzag
course in such cases. We do not think that such experiments
show that impulses do not usually follow a definite course, but
that the resources of nature are great, and that, when one tract
is not available, another is taken.
It is plain that impulses do not in any case travel by one and
the same nerve-fiber throughout the cord, for the size of this
organ does not permit of such a view being entertained ; at the
same time there is a relation between the size of a cross-section
of the cord at any one point and the number of nerves con-
nected with it at that region.
We may attempt to trace the paths of impulses in a cord
somewhat as follows: 1. Volitional impulses decussate chiefly
c
B
A\bs
1 V IV III II I V IV III II I XII XI X IX VIII VII VI V IV IU II I VIII VII VI V IV III II I
Sacral. Lumbar. Dorsal. Cervical.
Fig. 337.— Diagram to illustrate relative and absolute extent of (1) gray matter, (2)
white columns in successive sectional areas of spinal cord, and (31 sectional areas
of several nerve-roots entering cord. JVB, nerve roots; AC, LC, PC, anterior,
lateral, posterior columns; Gr, gray matter (after Schafer, Ludwig, and Woro-
schiloff).
in the medulla oblongata, but also, to some extent, throughout
the whole length of the spinal cord. They travel in the lateral
columns (crossed pyramidal tracts chiefly, if not exclusively),
and eventually reach the anterior roots of the nerves through
the anterior gray coruua, passing to them, possibly, by the ante-
rior columns. From the cells of the anterior cornua, impulses
414
COMPARATIVE PHYSIOLOGY.
travel by the anterior nerve-roots to the motor nerves, by
which connection is made with the muscles. 2. Sensory im-
pulses enter the cord from the afferent nerve-fibers by the pos-
3' 4 25 6 6 524 3 V
LOWER LIMIT OF
MEDULLA
4-
5"/' a
Fig. 338.— Diagram showing course of fibers in spinal cord (after Ranney). 1, 1', direct
pyramidal bundles; 2,2', crossed pyramidal bundles, decussating in medulla; 3.3',
direct cerebellar fibers; 4,4', fibers related to "muscular sense," decussating in
medulla; 5, 5', and 6,6', fibers relating to the appreciation of touch, pain, and
temperature. The motor bundles have a dot upon them to represent the motor
cells of the cord (anterior horn). Note that the motor fibers escape from the ante-
rior nerve-root (a. r.), and that the sensory bundles enter at the posterior nerve-
root (p. v.), which has a ganglion (rj) upon it.
terior nerve-roots, passing probably by the posterior columns to
the posterior cornua, thence to the lateral columns, decussation
being largely immediate though not completed for some dis-
tance up the cord.
It would seem that the lateral columns are the great high-
THE SPINAL CORD.— GENERAL. 475
ways of impulses ; though in all instances it is likely that the
gray matter of the cord plays an important part in modify-
ing them before they reach their destination. Some observers
believe that sensory impulses giving rise to pain travel by the
gray matter of the cord almost exclusively. It would be easy
to lay out the paths of impulses in a more definite and dog-
matic manner ; but the evidence does not seem to warrant it,
and it is better to avoid making statements that may require
serious modification, to say the least, in a few months. The
prominent principle to bear in mind seems to be that while
there are tracts in the cord of the animals that have been exam-
ined and probably of all that have well-formed spinal cords,
along which impulses travel more frequently and readily than
along others, it is equally true that these paths are not invaria-
ble, nor are they precisely the same for all groups of animals.
The cord can not be considered independently of the brain ; and
there can be no doubt that the paths of impulses in the former
are related to the constitution, anatomical and physiological, of
the latter. It is still a matter of dispute whether the cord is
itself irritable to a stimulus. As a whole it is without doubt ;
as also the white matter by itself. The gray matter is certainly
conducting, but whether irritable or not is still doubtful. Why
the sensibility of the side of the body on which one lateral half
of the cord has been divided should be increased (hyperesthe-
sia), is also undetermined. Possibly it is due to a temporary
disturbance of nutrition, or the removal of certain usual inhibi-
tory influences from above, either in the cord or brain.
THE AUTOMATIC FUNCTIONS OF THE SPINAL CORD.
Eeference has been already made to the fact that when por-
tions of a mammaFs cerebrum are removed the reflexes of the
cord become more pronounced, owing apparently to the removal
of influences operating on the cord from higher centers.
When the cord itself is completely divided across, it often
happens (in the dog, for example) that there are rhythmic
movements of the posterior extremities — i.e., when the animal
has recovered from the shock of the operation — that part of the
cord now independent of the rest and of the brain seems to
manifest an unusual automatism. The question, however, may
be raised as to whether this is a purely automatic effect, or the
result of reflex action. But, whichever view be entertained,
476 COMPARATIVE PHYSIOLOGY.
these phenomena certainly teach the dependence of one part
upon another in the normal animal, and should make one cau-
tious in drawing conclusions from any kind of experiment, in
regard to the normal functions. As we have often urged in
the foregoing chapters, what a part may under certain circum-
stances manifest, and what its behavior may he as usually
placed in its proper relations in the body, are entirely different,
or at least may be. When one leg is laid over the other and a
sharp blow struck upon the patella tendon, the leg is jerked up
in obedience to muscular contraction. It is not a little difficult
to determine whether this result is due to direct stimulation of
the muscle or to reflex action, the first link in the chain of
events necessary to call it forth originating in the tendon ;
hence the term tendon-reflex. But at present it is safer to
speak of it as the u knee-jerk," or the "tendon-phenomenon."
It disappears, however, when the spinal cord is destroyed or is
diseased, as in locomotor ataxia, or when the nerves of. the
muscles or the posterior nerve-roots are divided, showing that
the integrity of the center, the nerves, and the muscles are all
essential. There are normally many such phenomena (reflexes)
besides the "knee-jerk."
Another question very difficult to decide is that relating to
the usual condition of the muscles of the living animal. It is
generally admitted that the muscles of the body are all in a
somewhat stretched condition, but it is not so clear whether
the skeletal muscles are under a constant tonic influence like
those of the blood-vessels. It is certain that, when the nerves
going to a set of muscles are cut, when even the posterior roots
of the nerves related to the part involved are divided or the
spinal cord destroyed, there is an unusual flaccidity of the
limb involved. But the natural condition may be, it has been
suggested, the result of reflex action. The subject is probably
more complex than it has hitherto been considered.
The facts of such a case— those of the tendon-phenomenon
and similar ones — would be better understood if the spinal
cord, the nerves, and the muscles associated with them, were
regarded as parts of a whole so connected in their functions
that severance of any one of them leads to disorder of the rest.
That the cells of the cord are constantly exercising an influence
through the nerves on the muscles, while they in turn do not
lead an independent existence, but are as constantly influenced
by afferent impulses, and that one of the results is the condi-
THE SPINAL CORD.— GENERAL. 477
tion of the muscles referred to, is, we are convinced, the case.
To say that it is either entirely automatic or purely reflex, or
that the whole of the facts would be covered even by any com-
bination of these two processes, would probably be unjustifiable.
The influence of the centers over the metabolism of parts is
both constant and essential to their well-being ; and in such a
case as that now considered it may be that a certain degree of
tonus is normal to a healthy muscle in its natural surround-
ings in the body.
There is now considerable evidence in favor of placing cer-
tain centers presiding over the lower functions, as micturition,
defecation, erection of penis, etc., in the spinal cord of mam-
mals, especially its lower part — which centei^s, if they be not
automatic, are not reflex in the usual sense ; but their considera-
tion is better attempted in connection with the treatment of the
physiology of the parts over which they preside.
SPECIAL CONSIDERATIONS.
Comparative. — Among invertebrates there is, of course, no
spinal cord, but each segment of the animal is enervated by a
special ganglion (or ganglia) with associated nerves. Neverthe-
less, these are all so connected that there is a co-ordination,
though not so pronounced as in the vertebrate, in which the
actual structural bonds are infinitely more numerous, and the
functional ones still more so. From this result possibilities to
the vertebrate unknown to lower forms ; at the same time, in-
dependent life and action of parts are necessarily much greater
among invertebrates, as evidenced especially by the renewal of
the whole animal from a single segment in many groups, as in
certain divisions of worms, etc.
It also follows from the same facts that a vertebrated ani-
mal must suffer far more from injury, in consequence of this
greater dependence of one part on another ; a thousand things
may disturb that balance on which its well-being, indeed, its
very life hangs. It is noticeable, moreover, that, as animals
occupy a higher place in the organic scale, their nervous sys-
tem becomes more concentrated ; ganglia seem to have been
fused together, and that extreme massing seen in the spinal
cord and brain of vertebrates is foreshadowed. In the chapters
on the brain numerous illustrations of the nervous system in
lower forms will be found.
478 COMPARATIVE PHYSIOLOGY.
The fact that the brain and cord arise from the same germ
layer, and up to a certain point are developed almost precisely-
alike, is full of significance for physiology as well as morphol-
ogy. That original deep-lying connection is never lost, though
functional differentiation keeps pace with later morphological
differentiation. But even among vertebrates the spinal cord
shows a complexity gradually increasing with ascent in the
organic series. In the lowest of the fishes or vertebrates (Am-
phioxus lanceolatus) the creature possesses a spinal cord only
and no brain, so that an opportunity is afforded of witness-
ing how an animal deports itself in the absence of those direct-
ive functions, dependent on the existence of higher cerebral
centers. The Lancelet spends a great part of its life buried in
mud or sand on the bottom of the ocean, and its existence is
very similar to that of an invertebrate, though, of course, the
dependence of parts on each other is somewhat greater.
Evolution. — According to the general law of habit and in-
heritance, we should suppose that at birth each group of ani-
mals would manifest those reflex and other functions of the
cord which were peculiar to its ancestors. Observation and
experiment both show that reflexes, etc., are hereditary ; that
they tend to become more and more so with each generation ;
and at the same time that habit or exercise is essential for their
perfect development. They stand, in fact, in the same relation
as instincts, which are closely connected with them. Like the
latter, they may be modified by way of increase or diminution
and otherwise. To illustrate, it can not be doubted that gallop-
ing is the natural gait of horses, as shown by the tendency of
even good trotters to "break " or pass into a gallop ; but it is
equally well known that famous trotters breed trotters. In
other words, an acquired gait becomes organized in the nervous
system (especially) of the animal, and is transmitted with more
and more fixity and certainty with the lapse of time. But all
experience goes to show that walking, running, or any of the
movements of animals are, when fully formed as habit-reflexes,
dependent for their initiation on the will in most but not all
instances, and require for their execution certain combinations
of sensory and other afferent impulses, and the integrity of a
vast complex of nervous connections in the spinal cord.
It is well known that one in a period of absent-mindedness
will walk into a building to which he was accustomed to go
years before, though not of late, showing plainly that volition
THE SPINAL CORD.— GENERAL. 479
was not momentarily required for the act of walking and all else
that is involved in the above behavior. It suggests that certain
nervous and muscular connections have been formed, function-
ally at least. Plainly, then, we should not expect each indi-
vidual man's spinal cord to be the same, but that the series of
mechanisms of which every spinal cord is made up should differ
with experience ; and if this holds for individuals, how much
more must it be true of different groups of animals, the habits
of which differ so widely.
All the facts go to show that the cord is made up of nervous
mechanisms— if we may so speak — which are naturally associ-
ated, both structurally and functionally, with certain nerves
and muscles ; these, like the paths which impulses take to and
from the brain, though usual, are not absolutely fixed, though
more so as reflex than conducting paths, while they are con-
stantly liable to be modified in action by the condition of
neighboring groups of mechanisms, etc.
We have said less about the gray matter of the cord as a
conductor than its importance perhaps deserves. It is believed
by many that impulses which give rise to sensations of pain
always travel by the gray matter ; and there is not a little evi-
dence to show that, when none of the white columns are avail-
able, owing to operative procedure, disease, or other disabling
cause, the gray matter will conduct impulses that usually pro-
ceed by other tracts.
Synoptical. — The spinal cord is composed of large ganglionic
nerve-cells, fibers, and connecting neuroglia. Functionally it
is a conductor, the seat of certain automatic centers and of
reflex mechanisms. Probably in every case the one function is
to a certain extent associated with the other — i. e., when the
cord acts reflex! y it is also a conductor, and the cells concerned
are so readily excited to certain discharges of nervous energy
that automaticity is suggested, and so in other instances : thus,
in the case of automaticity, reflex influence or afferent impulses
are with difficulty entirely excluded from consideration.
The great majority of conducting fibers seem to cross either
in the cord itself or in the medulla oblongata. The conducting
paths that have been shown by pathological and clinical inves-
tigation to be best marked out in the spinal cord are those for
voluntary motor impulses. So far as the functions of the
human organ are concerned, clinical and pathological facts
have thrown the greatest amount of direct light on the subject;
480 COMPARATIVE PHYSIOLOGY.
but the inferences thus drawn have been modified and supple-
mented by the results of experiments on certain otber mam-
mals.
It is especially important to bear in mind that, while certain
conducting paths are usual, they are not invariable: in like
manner, reflex impulses may not be confined to usual groups of
cells, but may extend widely, and so briug into action a large
number of muscles. The resulting reflex in any case is depend-
ent on the character, intensity, and location of the stimulus,
and especially on the condition of the central cells involved.
In the whole functional life of the cord the influence of higher
centers in the organ itself and especially in tbe brain is to be
considered. The cord is rather a group of organs than a
single one.
THE BKAIN.
At the outset we may remark that the whole subject will
be studied more profitably if it be borne in mind that — 1. The
brain is rather a collection of organs, bound together by the
closest anatomical and physiological ties than a single one ; in
consequence of which it is quite impossible to understand the
normal function of one part without constantly bearing in
mind this relationship. This aspect of the subject has not re-
ceived the attention it deserves. No one regards the aliment-
ary tract as a single organ ; but it is likely that the dependence
functionally of one part of the digestive canal upon another
is not more intimate than that established in that great collec-
tion of organs crowded together and making up the brain. 2.
Since the relative size, position, and anatomical connections of
the parts that make up the brain are different in different
groups of animals, not to speak of the fact that the functions
of any part of the brain of an animal, like that of its spinal
cord, already alluded to, must depend in great part upon its
own and its inherited ancestral experiences, it follows that the
greatest caution must be exercised in applying conclusions true
of one group of animals to another. 3. It follows from what
has been referred to in 1 above, that conclusions based upon the
behavior of an animal after section or removal of a part of the
brain must be, until at least corrected by other facts, received
with some hesitation. 4. It also might be inferred from 1 that
it is desirable to study the simpler forms of brain found in the
lower vertebrates, in order to prepaid for the more elaborate
development of the encephalon in the higher mammals and in
man. 5. The embryological development of the organ also
thi'ows much light upon the whole subject.
The student will see from these remarks that a sound knowl-
edge of the anatomy of the brain and its connections is indis-
pensable for a just appreciation of its physiology; nor must
31
482 COMPARATIVE PHYSIOLOGY.
such knowledge be confined to any single form of the organ.
There is only one way by which this can be attained : dissection,
with the help of plates and descriptions. The latter alone fre-
quently impart ideas that are quite erroneous, though they
serve an especially good purpose in helping to fix the pictures
of the natural objects, and in reviving them when they have
become dim.
It is neither difficult to obtain nor to dissect the brain of the
fish, frog, bird, etc. Valuable material may be saved and the
subject approached profitably, if, prior to the dissection of a
human brain, a few specimens from some group or groups of the
domestic animals be examined. However useful artificial brain
preparations may be, they are so far from nature in color, con-
sistence, and many other properties, that, taken alone, they cer-
tainly may serve greatly to mislead ; and we hope the student
will allow us to urge upon him the methods above suggested
for getting real lasting knowledge. The figures given below
may prove helpful when supplemented as we advise.
The great difference in total size, and in the relative propor-
tion, situation, etc., of parts, will, however, be obvious, from the
figures themselves ; and as we have already pointed out more
than once, the preponderance of the cerebrum in man must
ever be borne in mind in the consideration of his entire organi-
zation,- whether physical, mental, or moral.
ANIMALS DEPRIVED OF THE CEREBRUM.
The cerebrum may be readily removed from a frog, without
producing either severe prolonged shock or any considerable
haemorrhage. Such an animal remains motionless, unless
wben stimulated, though in a somewhat different position from
that of a frog, having only its spinal cord. It can, however,
crawl, leap, swim, balance itself on an inclined plane, and when
leaping avoid obstacles. One looking at such an animal per-
forming these various acts would scarcely suspect that any-
thing was the matter with it, so perfectly executed are its move-
ments. We are forced to conclude, from its remaining quiet,
except when aroused by a stimulus, that its volition is lost; but,
apart from that, and the fact that it evidently does not see as
well as before, it appears to be normal. It has no intelligent
directive power over its movements. It remains, therefore, to
explain how it is that they are so much more complete, so
THE BRAIN. 483
much better co-ordinated in the entire animal than when only
the spinal cord is left. It seems to be legitimate to infer that
the other parts of the brain contain the nervous machinery for
this work, which is usually aroused to action by the will, but
which an external stimulus may render active. All the connec-
tions, structural and functional, are present, except those on
which successful volition depends. The frog with the cord
only, sinks at once when thrown into water; when gently
placed on its back, it may and probably will remain in that
position, without an attempt at recovery. There is, in fact,
very limited power of co-ordination.
Removal of the cerebral lobes in the bird is more likely to
be attended with difficulties, and conclusions must be drawn
with greater caution.
But a pigeon may be kept alive after such an operation for
months. It can stand, balancing on one leg ; recover its posi-
tion when placed on its side; fly when thrown into the air;
it will even preen its feathers, pick up food, and drink water.
Its movements are such as we might expect from a stupid, drow-
sy, or probably intoxicated bird ; but it is plainly endowed with
vision, though not as good as before. But spontaneous move-
ments are absent, and the pecking at food, etc., must be consid-
ered as associate reflexes, and as such are very interesting, in
that they show Iloav machine-like, after all, many of the appar-
ently volitional acts of animals really are. In a mammal so
great is the shock, etc., resulting from the operative procedure,
that the actual functions of the remaining parts of the brain,
when the cerebral convolutions are removed, are greatly ob-
scured ; nevertheless, little doubt is left on the mind that homol-
ogous parts discharge analogous functions. It can walk, run,
leap, right itself when placed in an unnatural position, eat when
food is placed in its mouth, and avoid obstacles in its path,
though not perfectly. Yet it remains motionless unless stimu-
lated ; all objects before its eyes impress it alike if at all. The
animal evidently has neither volition nor intelligence. Now, if
any of the parts between the cerebrum and the medulla be
removed the creature shows lessened co-ordinating power; so
that the inference that these various parts are essential constitu-
ents of a complex mechanism, all the components of which
are necessary to the highest forms of muscular co-ordination
and probably other functions, is unavoidable.
Since we are dealing with co-ordinated movements, we may
484 COMPARATIVE PHYSIOLOGY.
now treat of the functions of a portion of the ear, according to
our present classification.
HAVE THE SEMICIRCULAR CANALS A CO-ORDINAT-
ING FUNCTION?
Physiologists have as yet been unable to assign to the semi-
circular canals a function in hearing, and upon certain results,
partly of disease but chiefly of experiment, it has been con-
cluded, though somewhat dubiously, that they are concerned
with those sensations that conduce to or are essential to main-
tenance of the sense of equilibrium ; in a word, that they are
the organs of that sense in the same way that the eye is the
organ of vision.
Until further evidence is forthcoming, we are not inclined
to give assent to the existence of any mechanism in the semi-
circular canals, affording sensory data so entirely different
from those furnished by other recognized (and unrecognized)
sense-organs, that upon them alone, or in a manner entirely
their own, arises a consciousness of equilibrium. We are in-
clined to regard the latter as depending upon the fusion in con-
sciousness of a vast complex of sensations ; and that upon the
whole being there represented, or a portion wanting, depends
either the preservation of equilibrium, or a partial or entire loss
of the same. Nevertheless, it is highly probable that sensory
impulses of a very important character, in addition to such as
are essential for hearing, may proceed from the semicircular
canals, and indeed other parts of the labyrinth of the ear.
FORCED MOVEMENTS.
When certain portions of the brain of the mammal have
been injured, movements of a special character result, and, inas-
much as they are not voluntary, in the ordinary sense at least,
have been spoken of as forced or compulsory. The movements
may be classified according as they are around the long, the
vertical or the transverse axis of the body of the animal. Hence
there are " circus " movements, when the creature simply turns
about in a circle, " rolling " movements, etc. These and others
may be toward or from the side of injury. While in some
cases there may be a certain amount of muscular weakness in
consequence of the injury, which may, in part, account for the
THE BRAIN. 485
direction of the movements, this is not so in all cases ; nor does
it, in itself, explain the fact of their being plainly not volun-
tary in the usual sense.
The parts of the brain, which, when injured, are most liable
to be followed by forced movements are the basal ganglia (cor-
pora striata and optic thalami), the crura cerebri, corpora quad-
rigemina, pons Varolii, and medulla oblongata, and especially
if the section be unilateral. We have already seen that several
of these parts are concerned in muscular co-ordination ; hence
the disorderly character of any movements that might now re-
sult when any part of this related mechanism is thrown out of
gear, so to speak ; but, apart from that, we think that the view
presented in the previous sections is applicable in this case also,
while the forced movements themselves throw light upon the
symptoms following injury to the semicircular canals. When
that constant afflux of sensory impulses toward the nervous
centers is interfered with, as must be the case in such sections
as are now referred to, it is plain that the balance in conscious-
ness must be disturbed ; confusion results, and it is not sur-
prising that, instead of a passive condition, one marked by dis-
orderly movements should result in an animal, since movement
so largely enters into its life-habits. It is important to remem-
ber, in this connection, that the great highway of impulses
between tbe cerebral cortex and other parts of the brain and
the spinal cord lies in the very parts of the encephalon we are
now considering.
FUNCTIONS OP THE CEREBRAL CONVOLUTIONS.
Comparative.— It will conduce to the comprehension of this
subject if some reference be now made to the development of
the brain in the different groups of the animal kingdom.
Invertebi-ates not only have no cerebrum, but no brain in
the strict sense of the term as applied to the higher mammals.
In most forms of this great subdivision of the animal kingdom,
the first or head segment is provided with ganglia arranged in
the form of a collar around the oesophagus, by means of com-
missural nerve connections ; so that the nervous supply of the
head is not widely different from that of the other segments
of the body. But as we ascend in the scale among the in-
vertebrates these ganglia become more crowded together, and
so resemble the vertebrate brain with its massed ganglia and
486
COMPARATIVE PHYSIOLOGY.
numerous connections through nerve-fibers, etc. But in this
respect we find great difference among vertebrates. We can
recognize, on passing upward from the Amphioxus, destitute
of a brain proper, to man, all gradations in the form, relative
size, multiplicity of connecting ties, etc.
Speaking generally, there is great difference in the weight
of the cerebrum, both relative and absolute. In all animals be-
low the primates (man and the apes) the cerebellum is either
not at all or but imperfectly covered by the cerebrum ; wbile
Fig. 339. — Nervous system of medicinal leech (after Owen), a, double supra-oesopha-
geal ganglion connected with rudimentary ocelli (b, b) by nerves; c, double infra-
cesophageal ganglionic mass, which is. continuous with double ventral cord, hav-
ing compound ganglia at regular intervals.
in man, so great is the relative size of the latter, that the
cerebellum is scarcely visible from above. If we except the
elephant, in which the brain may reach the weight of ten
pounds, and the whale with its brain of more than five pounds
-a,
V
Fig 340.— Brain and cranial nerves of perch, seen from the side (after Gegenbaur and
Cuvier). A, cerebral lobe with olfactory ganglion in front; 7?, optic lobe; C, cerc-
■ bellum; />, medulla oblongata; I—VIIT, nerves in usual order; K, lateral branch
of vagns; I, upper twig of same; m, dorsal branch of trigeminus, joined by n, dor-
sal branch of vagus; o, /3, y, three branches of trigeminus; Se, facial nerve; \,
branchial branches of vagus.
THE BRAIN.
487
in the largest specimens, the brain of man is even absolutely
heavier than that of any other animal, which is in great part
due to the preponderating development of the cerebrum.
While the cerebral surface is smooth in all the lower verte-
brates, and but little convoluted until the higher mammals are
reached, the brain of the primates, and especially of man. has
its surface enormously increased, owing to its numerous fis-
sures and convolutions, which, in fact, arise from the growth
Fig. 341. — Brain and spinal cord of frog (Bastian). A, olfactory lobes; B, cerebral
lobes; R, pineal body; C, D, optic lobes; E, cerebellum; H, spinal cord. The
cerebellum is notably small.
of the organ being out of proportion to that of the bony case
in which it is contained ; and since those cells which go to
make up the gray matter and are devoted to the highest func-
tions, are disposed over the surface, the importance of the fact
in accounting for the superior intelligence of the primates,
Fig. 343.
Fh.o.-?
Fig. 343.
Fig. 342.— Brain of the pike, viewed from above (Huxley). ^4, the olfactory nerves or
lobes, and beneath them the optic nerves; B, the cerebral hemispheres; C, the
optic lobes; T). the cerebellum.
Fig. 343.— The t>rain of edible frog (Eana esculenta). 1x4. (After Huxley.) L.ol,
the rhinencephalon, or olfactory lobes, with 7, the olfactory nerves; He. the cere-
bral hemispheres; Fh.o, the thalamencephalon with the pineal gland, P/r, L.op,
optic lobes; 0, cerebellum; 8. rh, the fourth ventricle; Mo, medulla oblongata.
488
COMPARATIVE PHYSIOLOGY.
A
Fig. 344. — A, C, the brain of a lizard (Psammosaurus Bengalensis), and B, D, of a
bird (Meleagris gallopavo, the turkey), drawn as if they were of equal lengths
(after Huxley). A. B, viewed from above; C, D, from the left side. Olf, olfactory
lobes; Pn, pineal gland; Hmp, cerebral hemispheres; Mb, optic lobes of the mid-
brain; Cb, cerebellum; M. O, medulla oblongata; ii, iv, vi, second fourth, and
sixth pairs of cerebral nerves; Py, pituitary body.
Fri;. 8 1"). —Brains of a lizard (Psammosaurns Benr/alensifs) and of a bird {Meleagris
gallopava) in longitudinal and vertical section. The upper figure represents the
THE BRAIN.
489
lizard's brain; the lower, that of the bird (after Huxley and Carus). Letters as in
the preceding figure, except L. t, lamina terminaUs, or anterior wall of the third
ventricle; /. .)/, foramen of Munro; a, anterior commissure; Th. E, thalamen-
cephalon; s, soft commissure; p, posterior commissure; iv, indicates the exact
point of exit of the fourth pair from that part of the brain which answers to the
value of Vieussens.
and especially of man, becomes apparent. Depth of Assuring
is, however, of more importance than multiplicity of furrows ;
and it may be observed that intelligence is not always in pro-
portion to the extent to which the cerebral surface is broken
up into fissures and convolutions. Tbe depth of the gray mat-
ter is also very variable, and seems to bear an important rela-
tion to psychic development. Man's brain, then, is character-
ized by its great size and complexity ; while those parts treated
elsewhere, concerned in co-ordination, vision, etc., are well
developed, the cerebrum, especially its convolutions as distin-
guished from its basal ganglia, is, out of all proportion, greater
than in any other animal.
The gray matter of the brains of the higher vertebrates is
distributed as masses of ganglionic cells internally, and as a
fairly uniform layer over its surface. The brain of man weighs
about three pounds on the average, that of the male being
a few ounces (four to six) heavier than that of the female.
Fig. 346.
Fig. 347.
Fig. 348.
Fig. 346.— Brain of pigeon (after Ferrierl. A, cerebral hemispheres; B, optic lobe; C,
cerebellum, the lateral lobes of which are very small.
Fig. 347. — Brain and spinal cord of chick at sixteen days old; optic lobes, b, are still
in contact (after Owen and Anderson).
Fig. 348.— Brain and part of spinal cord of chick twenty days old, showing optic lobes
widely separated and cerebellum, c, largely developed.
The individual and race differences, though considerable, are
not comparable ha degree to those that distinguish man from
even the highest apes, the brain of the latter weighing not
more than about one third as much as that of the human sub-
ject. While it has been shown that individual men and women,
having brains of average or even sub-medium weight, may reach
490
COMPARATIVE PHYSIOLOGY.
even distinction in the intellectual world; and though idiots
have been known to possess brains abnormally heavy, it is
Pig. 350.
Fig. 349. — Outer surface of brain of horse (after Solly and Leuret). e, olfactory lobe;
h, hippocampal lobe (processus pyriformis); 1,2,3, lobes of cerebellum; o, optic
nerve; m, motor oculi; p, fourth nerve; t, fifth nerve; u, sixth nerve; /, facial;
I, auditory; r/, glossopharyngeal; v, vagus; s, spinal accessory; n, hypoglossal;
X, pons Varolii.
Pig. 350. — Longitudinal section through center of brain of horse, presenting view of
internal surface (after Solly and Leuret). c.c, corpus callosum; p, thalamus; co,
middle commissure; t. q, corpora quadrigemina, in front of which is the pineal
body. The cerebellum lias been cut through.
nevertheless true that brain-weight and the higher powers of
man bear a close though not invariable relationship. The
apparent discrepancies are susceptible of explanation.
Besides the gray matter, with its cells of highest functional
value from the standpoint now taken, the brain consists, and
in large part, of neuroglia and nerve-fibers, with probably
chiefly, and in the case of the fibers solely, a conducting func-
tion.
THE BRAIN.
491
The Connection of one Part of the Brain with another -
Though it has long been known that the different parts of the
suDra-orbit-il- 7 p \r j? t tr ,, „ ■ eyjviannssure, /». the insula; S. Or,
S. Oc, M. Oc, I. Oc, the three occipUal'gyri. ' ' ^ ^ three temPoraI> and
492
COMPARATIVE PHYSIOLOGY.
brain were connected by bridges of fibers (commissures, etc.),
tbe physiological significance of the fact seems to have been
largely ignored, and even at the present day is too little con-
PlG. 352.— Inner views of cerebral hemispheres of the rabbit, pig, and chimpanzee,
drawn as before, and placed in the same order (Huxley). 01, olfactory lobe; C.c,
corpus callosum ; A. C, anterior commissure; H, hippocampal sulcus; Vh, unci-
nate; M, marginal; 6', callosal gyri; /. P, internal perpendicular; C'a, calcarine;
Coll, collateral sulci; /'', fornix.
sidered. 1. Cerebral fibers pass between the convolutions of
this part of the brain and the cerebellum ; between the former
t-pr= "Y^y \"<^f
494
COMPARATIVE PHYSIOLOGY.
mr. J
Fig. 354.— Diagrammatic horizontal section of a vertebrate brain (Huxley). The follow-
ing letters serve for both this figure and the one following. Mb, mid-brain. What
lies in front of this is the fore-brain, and what lies behind, the hind-brain. L. t,
the lamina terminalis; Olf, olfactory lobes; limp, hemispheres; T/i. E, thala-
mencephalon; Pn, pineal gland; Py, pituitary body; FM, foramen of Munro; CS,
corpus striatum; Th, optic thalamus; CQ, corpora quadrigemina; CC, crura cere-
bri; Cb, cerebellum; PV, pons Varolii; MO, medulla oblongata; /, olf ac tori i; II,
optici; III, point of exit from brain of motores oculorum; IV, of pathetici; VI,
of abducentes; V—XII, origins of the other cerebral nerves. 1, olfactory ven-
tricle; 2, lateral ventricle; 3, third ventricle; 4, fourth ventricle; +, iter a tertio
ad quartum ventriculum.
and the main basal ganglia; between the gray matter of the
convolutions on the same side, and between the latter and those
Fio. 355.— A longitudinal and vertical section of a vertebrate brain (Huxley). Letters
as above. The lamina terminalis is represented by the strong black line between
FMimA Z.
THE BRAIN. 495
on the opposite halves; between the gray matter of the cortex
and the internal capsule, the corpora striata, optic thalami, pons
Varolii, the medulla oblongata, and so to the spinal cord. The
course of the latter tracts of fibers have been, especially by the
help of pathology, definitely followed. Some of these connec-
tions are given in more detail below.
1. Cerebro-cerebellar fibers, (a.) From the cortical cells of
the anterior cerebral lobe to tbe pons Varolii, passing through
the internal" capsule and thence through the lower and outer
part of the crus cerebri (crusta). (b.) Fibers from the occipital
and temporo-sphenoidal lobes, passing by the crusta, reach the
upper surface of the cerebellum.
2. Fibers bridging the tivo sides of the cerebrum, (a.) By
means of the corpus callosum chiefly, passing from the gray
matter in the first instance. (6.) From the temporo-sphenoidal
lobe on each side through the corpora striata and anterior com-
missure, (c.) Fibers from the upper part of the crus cerebri
(tegmentum) to the optic thalamus of each side and onward
to the temporo-sphenoidal lobes, forming the posterior com-
missure.
3. Fibers connecting different parts of the cerebral convolu-
tions on the same side. These are exceedingly numerous and
belong to such tracts as the "arcuate fibers," passing from one
gyrus to another; "collateral fibers," forming distant convo-
lutions; fibers of the fornix between the uncinate gyrus, hip-
pocampus major, and optic thalamus; longitudinal fibers of the
corpus callosum; fibers of the taenia semicircularis, uncinate
fasciculus, etc.
4. Fibers forming the cerebrum and the spinal cord. Ac-
cording as they pass downward or upward do they converge or
diverge, and the most important seem to pass through the in-
ternal capsule ; and while the majority do perhaps form some
connection either with the corpora striata and optic thalami,
some seem to pass directly downward through the internal cap-
sule. It is held by many that the fibers passing through the
posterior portion of the internal capsule are derived from the
posterior lobe of the cerebrum, and are the paths of sensory im-
pulses upward ; while the rest of the internal capsule is made
up of fibers from the anterior, and especially the middle portion
of the cerebral cortex (motor area), and these fibers are the
paths of motor (efferent) impulses.
It now becomes clearer that the brain is constituted a whole
496 COMPARATIVE PHYSIOLOGY.
by such connections ; and that, apart from the multiplicity of
cells with different functions to perform, situated in different
Fig. 356.— Diagrammatic representation of the course of some of the fibers in the cere-
brum of man (after Le Bon).
areas, the complexity and at the same time the unity of the
encephalon becomes increasingly evident, merely upon anatomi-
cal grounds; but we shall find such a view still further strength-
ened by study of the functions of the various parts. While the
tracts enumerated are anatomical and have been clearly traced,
there can be little doubt that many others yet remain to be
marked out ; and that, apart from such collections of fibers, we
must recognize functional paths by the neuroglia, and possibly
others still. It is not to be forgotten that in the brain, as in the
spinal cord, nerve-cells are themselves conductors, and while
THE BRAIX.
497
there may be certain areas within which the resistance is such
that impulses are usually confined to them, it is also true that,
as in the cord, there may be a kind of overflow. Adjacent cells,
possibly widely separated cells, may become involved. We shall
return to tbis important subject again, however, as, without
recognizing such relationships, it seems to us quite impossible
to understand the facts as we find them in the working of the
body and the mind.
The Cerebral Cortex. — We may now proceed to inquire what
are the functions of the cells of the gray matter covering the
surface of the cerebrum. Before the birth of physiology as a
32
49S
COMPARATIVE PHYSIOLOGY.
science, Gall recognized and taught that the encephalon is a col-
lection of organs ; that these have separate functions ; that the
relative size of each determines the degree of its functional ac-
tivity ; and that the cranium developing in proportion to the
growth of the brain, the former might give information as to
the probable size of what lay beneath it in different regions.
It will be seen that, as thus interpreted, phrenology is a very
different thing from what usually passes under that name, and
is paraded before wondering audiences by ignorant charlatans.
In the main the doctrines of Gall are not without a certain
foundation in facts ; and the modern theory of localization of
function bears some resemblance to what Gall taught, though
with greater limitations.
FlG. 358.— Outer surface of cerebrum (after Exner). The shaded portion represents
the motor area in man and the monkey— i. e., the area which most observers be-
lieve to be associated with certain voluntary movements of the limbs, etc.
In the mean time it has been found that in many cases it
was possible to locate the site of a brain-lesion (tumor, etc.) by
the symptoms, chiefly motor, of the patient ; and brain-surgery
THE BRAIN. 499
has in consequence entered upon a new era of development.
Tumors thus localized have been removed successfully, and the
patients restored to health. As a result of the various kinds of
observations and discussions on this subject of late years, the
localizationists are willing to admit that the areas of the cortex
can not be marked off mathematically — that, in fact, they
"overlap." This is in itself an important concession. Again,
there is less confidence in the location of the various sensory
centers than of the motor centers. Most investigators are be-
lievers in a " motor area " par excellence (for the arm, leg, etc.)
around the fissure of Eolando (Fig. 358). This view is now, so
far as man is concerned, widely accepted.
There is agreement in placing the sensory centers behind
the above-mentioned motor area, and especially in the occipital
lobes. The tendency to locate a visual center in this region is
growing stronger. There is much disagreement as to the other
sensory centers formerly placed in the angular gyrus and tem-
poro-sphenoidal lobes. The intellectual faculties have not been
located in any such sense as Gall and his followers attempted
to establish. The first two frontal convolutions are those, per-
haps, to which localization has as yet been least applied.
Chiefly on clinical and pathological grounds a center for
speech has long been located in the third (left) frontal convolu-
tion (Broca's) and parts immediately behind it. It has been ob-
served that when disease attacks this area speech is interfered
with in some way.
We may say then, generally, that the tendency at the pres-
ent time, both on the part of physiologists and clinical ob-
servers, is to admit localization to some degree and in some
sense. This has been the result in part of experiments on the
dog and especially on the monkey, combined with the discus-
sion of clinical cases which resulted in death (followed by an
autopsy), or of others marked by a successful diagnosis and re-
moval of lesions or other treatment. In other words, the truth,
if it is to be reached at all, must be sought by the plan we
have advocated throughout this work — the discussion of the re-
sult's of as inany different methods as can be brought to bear on
this or any other subject. Neither the experimental nor the
pathological method alone can settle such complex questions.
Although localization of function has not been established for
the cerebral cortex in the case of those animals with which the
practitioner of veterinary medicine has to deal as it has for man
500 COMPARATIVE PHYSIOLOGY.
and the monkey, we have thought it well to bring the subject
before the student of comparative medicine, since it can not be
doubted that future research will put the physiology of the
brains of the domesticated animals in a new light, in doing which
guidance will naturally be sought from what has been already
clone, more especially in the case of the human subject and his
nearest allies. Some would maintain that in the case of the
dog, motor and sensory localization has been established ; that
in this animal there is a motor area in the region of the crucial
sulcus corresponding to that around the fissure of Rolando in
man. The subject is, however, far from finally settled even in
the case of the dog, the brain of which has been more thoroughly
investigated than that of any other of our domestic animals.
Very little can as yet be said in regard to cortical localization
in the horse, ox, etc. It seems highly probable that investiga-
tion will show that cortical localization in the primates (man
and the monkey tribe) exists in a far higher degree than in
any other animals.
The Circulation in the Brain. — The brain, being inclosed
within an air-tight bony case, its circulation is of necessity
peculiar. Since any undue compression of the encephalon may
lead to even a fatal stupor, it is clear that there must exist some
provision to permit of the excess of arterial blood that is re-
quired for unusual activity of the brain. It is to be borne in
mind that the fluid within the ventricles is continuous, through
the foramen of Magendie in the roof of the fourth ventricle,
with that surrounding the spinal cord (spinal cavity) ; so that
an increase in the volume of the encephalon in consequence of
an afflux of blood might be in some degree compensated by an
efflux of the cerebro-spinal fluid. The part played by this ar-
rangement has, however, been probably overestimated. But
the peculiar venous sinuses do, it is likely, serve to regulate the
blood-supply ; being very large, they may answer as temporary
overflow receptacles. An inspection of the fontanelles of an
infant reveals a beating corresponding with the pulse; and,
when a large part of the cranium is removed in an animal, a
plethysmograph shows a rise in volume corresponding with
the pulse and the respiratory movements, as in the case of the
fontanelles. But, besides these, periodic waves of contraction
are now known to pass over the cerebral arteries.
Whether the latter is part of a general wave traversing the
whole arterial system is as yet uncertain. Though there is
THE BRAIN. 501
considerable anastomosis of vessels in the encephalon, it is not
equal to what takes place in many other organs. It is well
known that a clot or other plug within a cerebral vessel is more
serious than in many other regions, which is partly to be ex-
plained by the lack of sufficient anastomosis for the vascular
needs of the parts. It is also well known that, in organs which
constitute parts of a related series, as the different divisions of
the alimentary tract, all are not usually at the same time vas^
cular to the same extent. While they act functionally in rela-
tion to each other, they exemplify also a certain degree of inde-
pendence. Such a condition of things is now known to exist in
the brain — i. e., certain areas may be abundantly supplied with
blood as compared with others : and it seems highly probable
that a condition of equal arterial tension throughout is scarcely
a normal condition. Though the quantity of blood contained
within the vessels of the whole brain at any one time is not so
large as in some other organs (glands), yet the foregoing facts
and the rapidity of the flow must be taken into account. The
capillaries are very close and abundant, in the gray matter es-
pecially ; and it is to be borne in mind that it is chiefly these
vessels which are concerned in the actual metabolism (nutri-
tion) of parts. However, the chemical changes in the nervous
system being feeble, it would appear probable that it does its
work with less consumption of pabulum than other parts of
the body. We wish to lay stress on the local nature of vascular
dilatation in the brain, as it greatly assists in explaining certain
phenomena about to be considered.
Sleep. — Observations upon animals from which portions of
the cranium have been removed, so that the brain was visible,
show that during sleep the blood-vessels are much less promi-
nent than usual ; and it is well known that means calculated to
diminish the circulation in the brain, as cold and pressure, favor
sleep. It is also well established by general experience that
withdrawal of the usual afferent impulses through the various
senses favors sleep. A remarkable case is on record of a youth
whose avenues for sensory impressions were limited to one eye
and a single ear, and who could be sent to sleep by closing
these against the outer world. Yet this subject after a long
sleep would awake of his own accord, showing that, while affer-
ent impulses have undoubtedly much to do with maintaining
the activity of the cerebral centers, yet their automaticity (in-
dependence) must also be recognized.
502 COMPARATIVE PHYSIOLOGY.
It is a matter of common experience that weariness, or the
exhaustion following on pain, mental anxiety, etc., is favorable
to sleep.
A good deal of light is thrown on this subject by hiberna-
tion, particularly in mammals.
From special study of the subject we have ourselves learned
that, however temperature, and certain other conditions may
influence this state, it will appear at definite periods in defiance,
to a large extent, of the conditions prevailing. Hibernation,
we are convinced, is marked by a general slowing of all of the
vital processes in which the nervous system takes a prominent
part. Sleep and hibernation are closely related. In both there
is a diminution of the rate of the vital processes, as shown by
the income and output, measured by chemical standards, with
of course obvious physical signs, as slowed respiration, circula-
tion, etc. While sleep, then, is primarily the result of a rhyth-
mical retardation of the vital processes, especially within the
nervous system, it is like hibernation in some degree (in the
lowest creatures, without a nerve system) the outcome of that
rhythm impressed on every cell of the organism and the influ-
ence of which is felt in a thousand ways, that no doubt we are
quite unable to recognize.
Hypnotism. — By the help of the above principles the sub-
ject of hypnotism, now of absorbing interest, may be in great
part explained. This condition is characterized by loss of vo-
lition and judgment. It may be induced in man and certain
other animals by prolonged staring at a bright object, assisted
by a concentration of the attention on that alone, as far as pos-
sible, combined with a condition of mental passivity in other
respects. The individual gradually becomes drowsy, and finally
falls into a state in many respects strongly resembling sleep.
Hypnotism proper may be combined with catalepsy, a con-
dition in which the limbs remain rigid in whatever condition
tbey may be placed. Modifications of the vascular and respira-
tory systems occur. Various animals have been hypnotized, as
the fowl, rabbit, Guinea-pig, crayfish, frog, etc. This condi-
tion is readily induced in the common fowl, more especially
tbe wilder individuals, by holding the creature with the bill
down on a table and the whole animal perfectly quiet for a
short time. Upon the removal of the pressure the bird re-
mains perfectly passive and apparently asleep for some little
time.
THE BRAIN,
503
Fig. 359.— Lateral surface of brain of monkey, displaying motor areas (after Horsley
and Schafer).
Fk.. 860.- Median surface of brain of monkey (after Horsley and Schafer).
Figs. 359 and 300 may be said to embody the views of Horsley and Schafer more espe-
cially in regard to motor localization.
50A
COMPARATIVE PHYSIOLOGY.
FUNCTIONS OF OTHER PORTIONS OF THE BRAIN.
Certain parts of the encephalon are spoken of as the basal
ganglia, prominent among which are the corpus striatum and
the optic thalamus.
The Corpus Striatum and the Optic Thalamus.— The corpus
striatum consists of several parts, the main divisions being an
intra- ventricular portion or caudate nucleus, and an extra-ven-
tricular part or lenticular nucleus.
Fio. 361. — Transverse section of cerebral hemispheres of man at level of cerebral gan-
glia (after Dalton). 1, great longitudinal fissure; 2, part of same between occipital
lobes; 3, anterior part of corpus callosum; 4, fissure of Sylvius; 5, convolutions
of island of Rei) (insula); 6, caudate nucleus of corpus striatum; 7, lenticular nu-
cleus of corpus striatum; 8, optic thalamus; 9, internal capsule; 10, external cap-
sule; 11, claustrum.
THE BRAIN.
505
Between these lies the internal capsule, through which
pass fibers that spread out toward the cortex, as the corona
radiata.
Pathology, especially, has shown that a lesion of the intra-
ventricular portion of the corpus striatum, and, above all, of
the internal capsule, is followed by failure of voluntary move-
ment (akinesia). It would appear that a great part of the
fibers from the motor area around the fissure of Rolando, pass
through the intra-ventricular parts of the corpus striatum, and
especially its internal capsule. But it is also to be borne in
mind that a large part of the fibers passing from the cortex
make connection with the cells of the corpus striatum before
reaching the cord. These facts render the occurrence of loss of
voluntary motor power comprehensible.
The fibers of the peduncles of the brain may be divided into
an interior or lower division (cmista), going mostly to the
Fig. 362. — Transverse section of human brain (after Dalton). This and the preceding
figure are somewhat diagrammatic. 1, pons Varolii; -2,2. crura cerebri; 3,3, in-
ternal capsule; 4, 4, corona radiata; 5, optic thalamus; (j, lenticular nucleus; 7.
corpus callosum.
corpus striatum, and a posterior division (tegmentum), passing
principally to the optic thalami ; many, possibly most of them,
ultimately reach the cortex. Many clinical observers do not
hesitate to speak of the optic thalamus as sensory in function.
506 COMPARATIVE PHYSIOLOGY.
and the corpus striatum, as inotor ; but the clinical and patho-
logical evidence is conflicting — all lesions of these parts not
being followed by loss of sensation and motion respectively ;
though an injury to the internal capsule generally results in
paralysis. All are agreed that the symptoms are manifested on
the side of the body opposite to the side of the lesion, so that a
decussation must take place somewhere between the ganglion
and the periphery of the body.
There is no doubt that the optic thalamus, especially its
posterior part, is concerned with vision, for injury to it is fol-
lowed by a greater or less degree of disturbance of this func-
tion. As has been already pointed out, unilateral injury of
either of these ganglia leads to inco-ordination or to forced
movements. That these regions act some intermediate part in
the transmission of impulses to and from the brain cortex, and
that the anterior one is concerned with motor, and the pos-
terior possibly with sensory (tactile, etc.), and certainly with
visual impulses, may be stated with some confidence, though
further details are not yet a subject of general agreement.
Corpora Quadrigemina. — The function of these parts in vis-
ion, as in the co-ordination of the movements of the ocular
muscles, and their relations to the movements of the pupil, will
be considered later. However, the actual centers for these func-
tions seem to lie in the anterior portion of the floor of the
aqueduct of Sylvius, and are indirectly affected by stimulation
of the corpora quadrigemina. Extirpation of these parts on
one side produces blindness of the opposite eye, and in birds,
etc., the same result follows when their homologues — the optic
lobes — are similarily treated. There can be no doubt, therefore,
that they are a part of the central nervous machinery of vision,
and it seems to be probable that the anterior parts of the cor-
pora quadrigemina alone have this visual function. But, since
it is the opposite eye that is affected, and in some animals
(rabbits) that alone, we are led to infer a decussation of the
optic fibers, or at least of impulses. In dogs, on the other hand,
the crossing seems to be but partial.
It begins to appear that there are several parts of the brain
concerned with vision. After removal of almost any part of
the cerebral cortex, if of sufficient extent, vision is impaired.
We may say, then, that before an object is " seen " in the high-
est sense, processes beginning in the retina undergo further
elaboration in the corpora quadrigemina, optic thalami, and,
THE BRAIN.
507
finally in the cerebral cortex. We may safely assume that the
part played by the latter is of very great importance, making
the perception assume that highest completeness which is of
Pig. 303. — Diagrammatic representation of brain on transverse section to illustrate
course of fibers (after Landois). C, C, cortex cerebri; C.s, corpus striatum: X.I,
lenticular nucleus; T.o, optic thalamus; P. peduncle; H, tegmentum; j>. crusta;
V, corpora quadrigemina; 1. 1, corona radiata of corpus striatum; 2, 2, of 1cm icu-
lar nucleus; 3, 8, of optic thalamus; 4, 4, of corpora quadrigemina; 5, direct fibers
to cortex cerebri (Fleehsig); 6, 6, fibers from corpora quadrigemina to tegmentum;
m, further course of these fillers; S. s. tillers from corpus striatum and lenticular
nucleus to crusta of peduncle of cerebrum; M, further course of these; ■'\ 3, course
of sensory fibers; Ii, transverse section of spinal cord; v. W, anterior, and h. H".
posterior roots; a, a, system of association fibers; c. c, commissural fibers.
508 COMPARATIVE PHYSIOLOGY.
very varying character, no doubt, with different groups of ani-
mals. In a sense, all mammals may see alike, and, in another
sense, they may see things very differently ; for, if we may judge
by the differences in this respect between educated and unedu-
cated men, the great dissimilarity lies in the interpretation of
what is seen ; in a word, the cortex has to do with the perfect-
ing of visual impulses. Nevertheless, a break anywhere in the
long and complicated chain of processes must lead to some
serious impairment of vision. Much of the same sort of reason-
ing applies to the other senses and also to speech.
To speak, therefore, of a visual center or a speech center in
any very restricted sense is unjustifiable ; at the same time, it
is becoming clearer that there is in the occipital lobe, rather
than in other parts of the cortex, an area which takes a pecul-
iar and special share in elaborating visual impulses into visual
sensations and perceptions ; and there can be little doubt
that the other senses are represented similiarly in the cerebral
cortex.
The Cerebellum. — Both physiological and pathological re-
search point to the conclusion that the cerebellum has an im-
portant share in the co-ordination of muscular movements.
Ablation of parts of the organ leads to disordered movements ;
and, when the whole is removed in the bird, co-ordination is
all but impossible, and the same holds for mammals. Section
of the middle peduncle of one side is liable to give rise to roll-
ing forced movements. In fact, injury to the cerebellum causes
symptoms very similar to those following section of the semi-
circular canals, so that many have thought that in the latter
case the cerebellum had itself been injured.
Pathological. — Tumors and other lesions frequently, though
not invariably, give rise to unsteadiness of gait, much like that
affecting an intoxicated person. It may safely be said that the
cerebellum takes a very prominent share in the work of the
muscular co-ordination of the body.
As has already been pointed out, several tracts of the spinal
cord make connection with the cerebellum, and it is not to be
forgotten that this part of the brain has, in general, most ex-
tensive connections with other regions. Insufficient study has
as yet been given to the cerebellum, and it is likely that the
part it takes in the functions of the encephalon is greater than
has yet been rendered clear. The old notion that this organ
bears any direct relation to the sexual functions seems to be
THE BRAIN. 509
without foundation. It has now been clearly demonstrated
that the lower region of the spinal cord is, in the dog and prob-
ably most mammals, the part of the nerve-centers essential for
the sexual processes.
Crura Cerebri and Pons Varolii.— As has been already noted,
the peduncles (crura) are the paths of impulses from certain
parts of the cerebral cortex, the basal ganglia, and the spinal
cord. The functions of the gray matter of the crura are un-
known. But, since forced movements ensue on unilateral sec-
tion, it is plain that they also have to do with muscular co-
ordination.
The transverse fibers of the pons Varolii connect the two
halves of the cerebellum. Its longitudinal fibers have extensive
connections— the anterior pyramids and olivary bodies of the
medulla, the lateral, and perhaps also a part of the posterior
columns of the cord, while upward these fibers connect with
the crura cerebri and so with the cortex.
Pathological. — Paralysis of the face usually occurs on the
same side as that of the rest of the body ; hence it must be
inferred that there is a decussation somewhere of the fibers of
the facial nerve ; but there is much still to be learned about
this subject.
Medulla Oblongata. — In some animals (frogs) it is certainly
known that this region of the brain has a co-ordinating func-
tion, and it is probable that it is concerned with such uses in
all animals that possess the organ, or rather collection of organs,
seeing that this part of the brain must be regarded as especially
a mass of centers, the functions of which have been already
considered at length. So long as the medulla is intact, life may
continue ; but, except under special circumstances, which do
not invalidate this general statement, its destruction is followed
by the death of the animal.
We may simply enumerate the centers that are usually
located in the medulla : The respiratory (and convulsive), car-
dio-inhibitory, vaso-motor, center for deglutition, center for
the movements of the gullet, stomach, etc., and the vomiting
center ; center for the seci'etion of saliva and possibly other of
the digestive fluids. Some add a diabetic and other centers.
510
COMPARATIVE PHYSIOLOGY.
SPECIAL CONSIDERATIONS,
Embryological. — The further we progress in the study of the
nervous system, the greater the significance of the facts of its
Fig. 364.— Vertical longitudinal section of brain of human embryo of fourteen weeks.
1x3. (After Sharpey and Reichert.) c, cerebral hemisphere; cc, corpus callosum
beginning to pass back; /, foramen of Munro; p, membrane over third ventricle
and the pineal body; th, thalamus; 3, third ventricle; /. olfactory bulb ; cq, corpora
quadrigemina; cr, crura cerebri, and above them, aqueduct of Sylvius, still wide;
c', cerebellum, and below it the fourth ventricle; jw, pons Varolii; m, medulla
oblongata.
early development becomes. It will be remembered that from
that uppermost epiblastic layer of cells, so early marked off in
Fig. 366.
PlG. 365.— Outer surface of human frctal brain at six months, showing origin of prin-
cipal fissures (after Sharpey and R. Wagner). F, frontal lobe; P, parietal; 0,
occipital; T, temporal; a, a, a, faint appearance of several frontal convolutions;
g, 8, Sylvan fissure; *', anterior division of same; C, central lobe of island of Reil;
r, fissure of Rolando; p, external perpendicular fissure.
Pig. 366. Upper surface of bruin represented in Fig. 304 (after Sharpey and R. Wag-
ner).
the blastoderm, is formed *the entire nervous system, including
centers, nerves, and end organs. The brain may be regarded
as a specially differentiated part of the anterior region of the
THE BRAIN.
511
medullary groove and its subdivisions ; and the close relation
of the eye, ear, etc., to the brain in their early origin, is not
without special meaning, while the more diffused sensory de-
velopments in the skin connect the higher animals closely with
the lower — even the lowest, in which sensation is almost wholly
referable to the surface of the body.
Without some knowledge of the mode of development of
tbe encephalon, it is scarcely possible to appreciate that rising
grade of complexity met with as we pass from lower to higher
groups of animals, especially noticeable in vertebrates ; nor is
it possible to recognize fully the evidence found in the nervous
system for the doctrine that higher are derived from lower
forms by a process of evolution.
Evolution. — The same law applies to the nervous system as
to other parts of the organism, viz., that tbe individual devel-
opment (ontogeny) is a synoptical representation, in a general
way. of the development of the group (phylogeny). A com-
parison of the development of even man's brain reveals the fact
that, in its earliest stage, it is scarcely, if at all, distinguishable
from that of any of the lower vertebrates. There is a period
when even this, the most convoluted of all brains, is as smooth
and devoid of gyri as the brain of a frog. The exti'erne com-
PlG. 367.— A. brain of aye-aye {Lemur); B. of marmoset; C. of squirrel monkey (Cal-
lithri.c); D, of macaque monkey; E, of gibbon; F, of a fifth-month human foetus
(after ( >wen i. Although naturalists are agreed that tbe monkey.-, apes, and lemurs
are related, considerable differences are to be observed in their brains. These fig-
ures also illustrate the remark made after those following.
plexity of the human brain is referable to excessive growth of
certain parts, crowding and alteration of shape, owing to the
influence of its bony case, its membranes, etc.
512
COMPARATIVE PHYSIOLOGY.
It is evident, from an inspection of the cranial cavities of
those enormous fossil forms that preceded the higher verte-
brates, that their brains, in proportion to their bodies, were
very small, so that any variation in the direction of increase
in the encephalon — especially the cerebrum — must have given
the creatures, the subject of such variation, a decided advan-
tage in the struggle for existence, and one which may partly
account, perhaps, for the extinction of those animals of vast
proportions but limited intelligence. That the size of the brain
Fig. 368. — A, brain of a chelonian; B, of a foetal calf; C, of a cat. (All after Gegen-
baur.) /, indicates cerebral hemispheres ; //, thalamus ; III, corpora quadri-
gemina; IV, cerebellum; V, medulla; st, corpus striatum; /, fornix; h, hippocam-
pus; sr, fourth ventricle; g, geniculate body; ol, olfactory lobe. It will be observed
(1) how the foetal brain in a higher animal form resembles the developed brain in
a lower form, and (2) how certain parts become crowded together and covered
over by more prominent regions, e. g., the cerebrum, as we ascend the animal scale.
as well as its quality can be increased by use, seems to have
been established by the measurements, at different periods of
development, of the heads of those engaged in intellectual pur-
suits, and comparing the results with those obtained by similar
measurement of the heads of those not thus specially employed.
Of course, it must be assumed that the head measurement is a
gauge of the size of the brain, which is approximately true, if
not entirely so.
Recent investigations seem to show that the development
of the ganglion cells of the brain takes place first in the me-
dulla, next in the cerebellum, after that in the mid-brain, and
finally in the cerebral cortex. Animals most helpless at birth
are those with the least development of such cells. The me-
THE BRAIN.
513
dulla may be regarded in some sense as the oldest (phylogeneti-
cally) part of the brain. In it are lodged those cells (centers)
which are required for the maintenance of the functions essen-
tial to somatic life. This may serve to explain how it is that
so many centers are there crowded together. It is remarkable
that so small a part of the brain should preside over many im-
portant functions ; but the principle of concentration with pro-
gressive development, and the law of habit making automatism
prominent, throw some light upon these facts, and especially
the one otherwise not easy to understand, that so much impor-
tant work should be done by relatively so few cells. Possibly,
however, if localization is established as fully as it may eventu-
ally be, this also will not be so astonishing.
The law of habit has, in connection with our psychic life
and that of other mammals, some of its most striking develop-
ments. This has long been recognized, though that the same
law is of universal application to the functions of the body has
as yet received but the scantiest acknowledgment.
We shall not dwell upon the subject beyond stating that in
our opinion the psychic life of animals can be but indifferently
understood unless this great factor is taken into the account ;
and when it is, much that is apparently quite inexplicable be-
comes plain. That anything that has happened once any-
where in the vital economy is liable to repetition under a
Fig. 369.
Fig. 370.
Fig. 369.-
Fig. 370.-
-Brnin of cat, seen from above (after Tiedemann).
-Brain of dog, seen from above (after Tiedemann).
slighter stimulus, is a law of the utmost importance in physiol-
ogy, psychology, and pathology. The practical importance of
this, especially to the young animal, is of the highest kind.
Synoptical.— There is as yet no systematized clear physiology
514 COMPARATIVE PHYSIOLOGY.
of "the brain." We are conversant with certain phenomena
referable to this organ in a number of animals, chiefly the
higher mammals ; but our knowledge is as yet insufficient to
generalize, except in the broadest way, regarding the functions
of the brain — i.e., to determine what is common to the brains of
all vertebrates and what is peculiar to each group. Referring,
then, to the higher mammals, especially to the dog, the cat, the
monkey, and man, we may make the following statements :
The medulla oblongata is functionally the ruler of vegeta-
tive life — the lower functions ; and so may be regarded as the
seat of a great number of " centers," or collections of cells with
functions to a large degree distinct, but like close neighbors,
with a mutual dependence.
Phylogenetically (ancestrally) the medulla is a very ancient
region, hence the explanation apparently of so many of its
functions being common to the whole vertebrate group.
Parts of the mesencephalon, the pons Varolii, the optic lobes
or corpora quadrigemina, the crura cerebri, etc., are not only
connecting paths between the cord and cerebrum, but seem to
preside over the co-ordination of muscular movements, and to
take some share in the elaboration of visual and perhaps other
sensory impulses.
The cerebellum may have many functions unknown to us.
Its connections with other parts of the nerve-centers are numer-
ous, though their significance is in great part unknown. Both
pathological and physiological investigation point to its having
a large share in muscular co-ordination.
It is certain that the cerebrum is the part of the brain essen-
tial for all the higher psychic manifestations in the most ad-
vanced mammals and in man.
The preponderating development of man's cerebrum ex-
plains at once his domination in the animal world, his power
over the inanimate forces of Nature, and his peculiar infirmities,
tendencies to a certain class of diseases, etc., — in a word, man is
man, largely by virtue of the size and peculiarities of this part
of his brain.
Modern research has made it clear also that there is a " pro-
jection " of sensory and motor phenomena in the cerebral cor-
tex ; in other words, that there are sensory and motor centers
in the sense that in the cortex there are certain cells which have
an important share in the initiation of motor impulses, and
others employed in the final elaboration of sensory ones.
THE BRAIN. 515
It is even yet premature to dogmatize in regard to the site
of these centers; especially are we not ready for large generali-
zations. In man the convolutions around the fissure of Rolando
constitute the motor area hest determined.
The whole subject of cortical localization requires much ad-
ditional study, especially by the comparative method in the
widest sense — i. e., by a comparison of the results of operative
procedure in a variety of groups of animals, and the results of
clinical, pathological, physiological, and psychological investi-
gation. Especially must allowance be made for differences to
be observed, both for the group and the individual ; and also
for the influence which one region exerts over another. Be-
tween the weight of the cerebrum, the extent of its cortical
surface, and psychic power, there is a general relationship.
GENERAL REMARKS ON THE SENSES.
Oue studies in embryology have taught us that all the vari-
ous forms of end-organs are developed from the epiblast, and
so may be regarded as modified epithelial cells, with which are
associated a vascular and nervous supply. These end-organs
are at once protective to the delicate nerves which terminate in
Fig. 371.— Papillae of skin of palm of hand (after Sappey). A vascular network in all
cases, and in some nerves and tactile corpuscles, enter the papillae.
them, and serve to convey to the latter peculiar impressions
which are widely different in most instances from those result-
ing from the direct contact of the nerve with the foreign body,
All are acquainted with the fact that, when the epithelium is
removed, as by a blister, we no longer possess tactile sensibility
of the usual kind, and experience pain on contact with objects ;
in a word, the series of connections necessary to a sense-percep-
tion is broken at the commencement.
Seeing that all the end-organs on the surface of the body
have a common origin morphologically, it would be reasonable
to expect that the senses would have much in common, espe-
cially when these organs are all alike connected with central
nervous cells by nerves. As a matter of fact, such is the case,
GENERAL REMARKS ON THE SENSES.
517
and in every instance we can distinguish between sensory im-
pulses generated in the end-organ, conveyed by a nerve inward,
and those in the cells of these central nervous systems, giving
rise to certain molecular changes
which enable the mind or the
ego to have a perception proper ;
which, when taken in connec-
tion with numerous past experi-
ences of this and other senses,
furnishes the material for a sen-
sory judgment.
The chief events are, after
all, internal, and hence it is
Fig. 372.
Fig 373.
Fig. 372.— Corpuscle of Vater (after Sappey).
Fig. 373. — End-bulbs (corpuscles) of Krause (after Luddon). A. from conjunctiva ot
man; B, from conjunctiva of calf. It may be noticed that in all these cases the
nerve loses its non-essential parts before entering the corpuscle.
found that the higher in the scale the animal ranks, the more de-
veloped its nervous centers, especially its brain, and the more it
is able to capitalize its sensory impulses ; also the greater the de-
gree of possible improvement by experience, a difference well
seen in blind men whose ability to succeed in life without vision
is largely in proportion to their innate and acquired mental
powers. Inasmuch as all cells require rest, one would expect
518
COMPARATIVE PHYSIOLO -Y.
that under eonstaut stimulation fatigue w. raid soon result ana
perceptions be imperfect Hence it happens that all the senses
fail when exercised.
G
\
even for but a short
period, without change
of stimulus lead:: _
alteration of condition
in the central cells,
The change need not
be one of entire rest.
but merely a new form
of exercise. Hence the
freshness experienced
by a change of view on
passing through beau-
tiful scenery.
Exhaustion may
not he confined wholly
to the central nerve-
Fig. 374.— Nerves with sranglion cells {G) beneath a cells, but there Can be
$&?Jw*'- fr°m Skin °f "m arthropod little doubt that they
are the most affected.
Since also there must be a certain momentum, so to speak, to
molecular activity, it is not surprising that we find that the
sensation outlasts the stimulus for a brief period: and this ap-
plies to all the senses, and necessarily determines the rapidity
with which the successive stimuli may follow each other with-
out causing a blending of tne sensations.
Thus, then, in every sense
organ in which the chain of pi
nerve through which (3 the central near
and we may speak, therefore, of (1) senso
sensations, when these give rise t<-> atf'ect
nervous cells resulting in 1 perceptions
when we take into account the psychic i r
nature of cell-life generally, we must reco£
sity of the stimulus ry to arouse a sensation and a limit
within which alone we have power to discriminate (range of
stimulation and perception): and also a limit to the ra
with which stimuli may succeed each other to any advantage,
to give ris rsens is; and a limit to the endur-
ance of the apparatus in good working condition corresponding
must recognize 1< an end-
ses begins; (2) a conducting
e-cells are affected ;
y impulses and 2
ons of the central
and •„' jui a.
e-^e> : and. from the
a certain inten-
GENERAL REMARKS ON THE SENSES. 519
to clear mental perceptions, together with the value of past ex-
perience in the interpretation of our sensations. A man can
necessarily have positive knowledge only of his own conscious-
ness ; but he infers similarity of conscious states by likeness in
action and expression in his fellows. It is by an analogous
process and by such alone that we can draw any conclusions in
regard to the sensations of the lower animals. The presence of
structures, undoubtedly sensory, in them is fairly good evidence
that their sensations resemble ours when similar organs are em-
ployed. However, this does not absolutely follow; and the
whole subject of the senses of animals incapable of articulate
speech is beset witb great difficulties. It only remains for us to
set forth what is known regarding man, assuming that at
least much of it applies to our domestic animals. Patient
thoughtful observation will in time place the subject in a better
position.
THE SKIN AS AN ORGAN OF SENSE.
Bearing in mind that all the sensory organs originate in
the ectoderm, we find in the skin even of the highest animals
the power to give the central nervous system such sense-im-
pressions as bear a relation to the original undifferentiated
sensations of lower forms as derived from the general surface
of the body, but with less of specialization than is met with in
the sense of hearing and vision, so that it is possible to under-
stand how it is that the skin must be regarded not only as the
original source of sensory impulses for the animal kingdom,
but why it still remains perhaps the most important source of
information in regard to the external world, and the condition
of our own bodies; for it must be remembered that the data
afforded for sensory judgments by all the other senses must
be interpreted in the light of information supplied by the skin.
We really perceive by the eye only retinal images. The dis-
tance, position, shape, etc., of objects are largely determined by
feeling them, and thus associating with a certain visual sensa-
tion others derived from the skin and the muscles, which latter
are, however, generally also associated with tactile sensations.
It is recorded of those blind from birth that, when restored
to sight by surgical operations, they find themselves quite un-
able to interpret their visual sensations; or, in other words,
seeing they do not understand, but must leaim by the other
senses, especially tactile sensibility, what is the real nature of
the objects that form images on their retinae. All objects seen
appear to be against the eyes, and any idea of distance is out of
the question.
Special forms of end-organs are found scattered over the
skin, mucous and serous surfaces of the body, such as Pacinian
corpuscles, touch-corpuscles, end-bulbs, etc. ; while in lower
forms of vertebrates many others are found in parts where sen-
sibility is acute. There seems to be little doubt that these are
THE SKIN AS AN ORGAN OF SENSE. 521
all concerned with the various sensory impulses that originate
in the parts where they are found, but it is not possible at pres-
ent to assign definitely to each form its specific function.
It has been contended that the various specific sensations
of taste, as bitter, sweet, etc., are the result of impulses con-
veyed to the central nervous system by fibers that have this
function, and no other; and a like view has been maintained
for those different sensations that originate from the skin.
For such a doctrine there is a certain amount of support from
experiment as well as analogy ; but the more closely the subject
is investigated the more it appears that the complexity of our
sensations is scarcely to be explained in so simple a way as
many of these theories would lead us to believe. Whether
there are nerve-fibers with functions so specific, must be re-
garded as at least not yet demonstrated.
Let us now examine into the facts. What are the different
sensations, the origin of which must be in the first instance,
sought in the skin, as the impulses aroused in some form of
end-organ or nerve-termination ?
Suppose that one blindfolded lays his left hand and arm
on a table, and a piece of iron be placed on the palm of his
hand, he may be said to be conscious of the nature of the sur-
face, whether rough or smooth, of the form, of the size, of the
weight, and of the temperature of the body: in other words,
the subject of the experiment has sensations of pressure, of
tactile sensibility, and of temperature at least, if not also to
some extent of muscular sensibility. But if the right hand be
used to feel the object its form and surface characters can be
much better appreciated ; while, if the body be poised in the
hand, a judgment as to its weight can be formed with much
greater accuracy. The reason of the former is to be sought in
the fact that the finger-tips are relatively very sensitive in
man, and that from experience the mind has the better learned
to interpret the sensory impulses originating in this quarter;
which again resolves itself into the particular condition of the
central nerve-cells associated with the nerve-fibers that convey
inward the impulses from those regions of the skin. Mani-
festly if there be a sense referable to the muscles (muscular
sense) at all, when they are contracted at will the impression
must be clearer than when they but feebly respond to the mere
pressure of some body.
522 COMPARATIVE PHYSIOLOGY.
PRESSURE SENSATIONS.
1. There is a relation between the intensity of the stimulus
and the sensation resulting, and this limit is narrow. The
greater the stimulus the more pronounced the sensation, though
ordinary sensibility soon passes into pain. 2. The law of con-
trast may be illustrated by passing the finger up and down in a
vessel containing mercury, when the pressure will be felt most
distinctly at the point of contact of the fluid. 3. Pressure is
much better estimated by some parts than others ; bence the use
of the employment of those to so large an extent.
THERMAL SENSATIONS.
1. The law of contrast is well illustrated by this sense ; in
fact, the temperature of a body exactly the same as that of the
part of the skin applied to it can scarcely be estimated at all.
The first plunge into a cold bath gives the impression that the
water is much colder than it seems in a few seconds after, when
the temperature has in reality changed but little ; or, perhaps,
the subject may be better illustrated by dipping one hand into
wanner and the other into colder water than that to be ad-
judged. The sample feels colder than it really is to the hand
that has been in the warm water, and warmer than it is to the
other. 2. The limit within which we can discriminate is at most
small, and the nicest determinations are made within about 27°
and 33° C. — i. e., not far from the normal temperature of the
body. 3. Variations for the different parts of the skin are
easily ascertained, though they do not always correspond to
those most sensitive to changes in pressure. The cheeks, lips,
and eyelids are very sensitive to pressure.
Recent investigations have revealed the fact that there are in
the human skin " pressure-spots,'' and " cold-spots," and " heat-
spots"— i. e., the skin may be mapped out into very minute
areas which give when touched a sensation of pressure different
from that produced by the same stimulus in the intermediate
regions ; and in like manner are there areas which are sensitive
to warm and to cold bodies respectively, but not to both ; and
these do not correspond with the pressure-spots, nor to those
that give rise when touched to the sensation of pain.
It has been shown, also, that the extent of the area of skin
stimulated detennines to a large degree the quality of the re-
THE SKIN AS AN ORGAN OF SENSE. 523
suiting sensation. Thus, the temperature of a fluid does not
seem the same to a finger and the entire hand. This fact is not
irreconcilable with the existence of the various kinds of ther-
mal spots, referred to above, but it does re-enforce the view we
are urging of the complexity of those sensations which seem to
us to form simple wholes — as, indeed, they do — just as a piece
of cloth may be made up of an unlimited number of different
kinds of threads.
TACTILE SENSIBILITY.
As a matter of fact, one may learn, by using a pair of com-
passes, that the different parts of the surface of our bodies are
not equally sensitive in the discrimination between the contact
of objects — i. e., the judgment formed as to whether at a given
instant the skin is being touched by one or two points is de-
pendent on the part of the body with which the points are
brought into contact.
Certain it is that exercise of these and all the senses greatly
improves them, though it is likely that such advance must be
referred rather to the central nerve-cells than to the peripheral
mechanism.
We practically distinguish between a great many sensations
that we can neither analyze nor describe, though the very
variety of 'names suffices to show how much our interpretation
of sense depends on past experience.
Mammals are always able to define the part of their bodies
touched, and with great accuracy, no doubt, owing to the simul-
taneous use in the early months and years of life of vision and
the senses resident in the skin.
An impression made on the trunk of a nerve is referred to
the peripheral distribution of that nerve in the skin ; thus, if
the elbow be dipped in a freezing mixture, the skin around the
joint will experience the sensation of cold, but a feeling of pain
will be referred to the distribution of the ulnar nerve in the
hand and arm. The same principle is illustrated by the com-
mon experience of the effects of a blow over the ulnar nerve,
the pain being referred to the peripheral distribution ; also by
the fact that pain in the stump of an amputated limb is thought
to arise in the missing toes, etc.
524 COMPARATIVE PHYSIOLOGY.
THE MUSCULAR SENSE.
Every one must be aware how difficult it is to regulate his
movements when the limbs are cold or otherwise deadened in
sensibility. We know too that, in judging of the muscular
effort necessary to be put forth to accomplish a feat, as throw-
ing a ball or lifting a weight, we judge by our past experience.
It is ludicrous to witness the failure of an individual to pick up
a mass of metal which was mistaken for wood. In these facts
we recognize that in the successful use of the muscles we are
dependent, not alone on the sensations derived from the skin,
but also from the muscles themselves. True, the muscles are
not very sensitive to pain when cut; it does riot, however, fol-
low that they may not be sensitive to that different effect, their
own contraction. Whether the numerous Pacinian bodies
around joints, or the end-organs of the nerves of muscles are
directly concerned, is not determined.
Pathological. — The teaching of disease is plainly indicative
of the importance of sensations derived both from the skin and
the muscles for co-ordination of muscular movements.
In locomotor ataxy, in which the power of muscular co-
ordination is lost to a large extent, the lesions are in the pos-
terior columns of the spinal cord, or the posterior roots of the
nerves, or both, and these are the parts involved in the trans-
mission of afferent impulses.
Comparative. — The more closely the higher vertebrates are
observed, the more convinced does one become that those sen-
sory judgments, based upon the information derived from the
skin and muscles, which they are constantly called upon to form
are in extent, variety, and perfection scarcely if at all surpassed
by those of man. Of course, sensory data in man, with his ex-
cessive cerebral development, may by associations in his expe-
rience be worked up into elaborate judgments impossible to the
brutes, but we now refer to the judgments of sense in them-
selves.
The lips, the ears, the vibrissa? or stiff hairs, especially about
the lips, the nose, in some cases the paws, all afford delicate and
extensive sensory data.
It is a remarkable fact that the most intelligent of the
groups of animals have these sensory surfaces well developed,
as witness the elephant with his wonderful trunk, the band of
the monkey, and the paws and vibrissa? of the cat and dog tribe.
THE SKIN AS AN ORGAN OF SENSE. 525
On the other hand, the groups with hoofs are notably inferior
in the mental scale. When we pass to the lower forms of in-
vertebrates the appreciation of vibrations of the air or water
in which they live, of its temperature, of its pressure, etc., must
be considerable to enable them to adapt themselves to a suitable
environment.
We have not spoken of sensations derived from the internal
organs and surfaces. These are ill-defined, and we know them
mostly either as a vague sense of comfort or discomfort, or as
actual pain. We are quite unable to refer them at present to
special forms of end-organs. They are valuable as reports and
warnings of the animal's own conditions.
After-impressions (" after-images") of all the senses referred
to exist, mostly positive in nature— i. e., the sensation remains
when the stimulus is withdrawn.
Synoptical. — The information derived from the skin in man
and the other higher vertebrates relates to sensations of press-
ure, temperature, touch, and pain. The muscles also supply
information of their condition. In how far these are referable
to certain end-organs in the skin is uncertain. There are der-
mal areas that give rise to the sensations of heat, cold, pressure,
and pain. Whether these are connected with nerve-fibei's that
convey no other forms of impulses than those thus arising is
undetermined .
In all these senses the laws of contrast, duration of the im-
pression, limit of discrimination, etc., hold.
The judgments based on sensations derived from the skin
are syntheses or the result of the blending of many component
sensations simultaneous in origin. All our sensory judgments
are very largely dependent on our past experience.
VISION.
Light and vision are to some degree correlatives of each
other. Light is supposed to have as its physical basis the vibra-
tions of an imponderable ether. Such is, however, to a non-
seeing animal as good as non-existent, so that we may look at
15 13 16
Fig. 37.").— Eye partially dissected (after Sappeyl. 1. optic nerve; 2, 3, 4, sclerotic dis-
sected back so as to uncover the choroid coat; 5, cornea, divided and folded back
with sclerotic coat; 6, canal of Schlemm; 7, external surface of choroid, traversed
by one of the long ciliary arteries and by ciliary nerves; 8. central vessel, into
which the vam vorticoea empty; 9, 10. choroid zone; 11, ciliary nerves; 12, long
ciliary artery; 13, anterior ciliary arteries; 14, iris; 15, vascular circle of iris; 16,
pupil.
this subject either with the eyes of the physiologist or the phys-
icist, according as we regard the cause of the effects or the
latter and their relations to one another. It is, however, im-
possible to understand the physiology of vision without a
sound knowledge of the anatomy of the eye, and an apprehen-
VISION.
527
sion of at least some of the laws of the science of optics. The
student is, therefore, recommended to learn practically the
SUPERIOR RECTUS
CHOROID
OPTIC NERVE
-INFERIOR RECTUS
Fig. 376.— Section of human eye, somewhat diagrammatic (after Flint)
coarse and microscopic structure of the eye in detail. The eyes
of mammals are sufficiently alike to make the dissection of any
of them profitable. Bullocks' eyes are readily obtainable, and
from their large size may be used to advantage. We recom-
mend one to be boiled hard, another to be frozen, and sections
in different meridians to be made, especially one axial vertical
longitudinal section. Other specimens may be dissected with
and without the use of water.
Assuming that some such work has been done, and that the
student has become quite familiar with the general structure
of the eye, we call attention specially to the strength of the
sclerotic coat ; the great vascularity of the choroid coat and its
terminal ciliary processes, its pigmented character adapting it
for the absorption of light; the complicated structure and pro-
tected position of the retinal expansion. It may be said that
528
COMPARATIVE PHYSIOLOGY.
the whole eye exists for the retina, and that the entire mechan-
ism besides is subordinated to the formation of images on this
Fig. 377. — Certain parts of eye. 1 x 10. (After Sappey.) 1, 1, crystalline lens; 2,
hyaloid membrane; 3, zonule of Zinn; 4, iris; 5, a ciliary process; 6, radiating
fibers of ciliary muscle; 7, section of circular portion of ciliary muscle; 8, venous
plexus of ciliary muscle; 9, 10, sclerotic coat; 11, 12, cornea; 13, epithelial layer of
cornea; 14. Descemet's membrane; 15, pectinate ligament of iris; 16, epithelium
of membrane of Descemet; 17, union of sclerotic coat with cornea; 18, section of
canal of Schlemm.
nervous expansion. The eye of the mammal may be regarded
as an arrangement of refracting media, protected by coverings,
with a window for the admission of light, a curtain regulating
the quantity admitted; a sensitive screen on which the images
are thrown ; surfaces for the absorption of superfluous light ;
apparatus for the protection of the eye as a whole, and for pre-
serving exposed parts moist and clean.
Embryological. — We have already learned that the first indi-
cation of the eye is the formation of the optic vesicle, an out-
growth from the first cerebral vesicle. This optic vesicle be-
VISION.
329
comes more contracted at the base, and the optic stalk remains
as the optic nerve.
At an early stage of development (second or third day in the
chick) the outer portion of the optic vesicle is pushed inward,
Fig. 379.
Fig. 378.
Fig. 378. — Section through head of chick on third day, showing origin of eye (after
Yeo). a, epiblast undergoing thickening to form lens; o, optic vesicle; Vx, first
cerebral vesicle; V2, posterior cerebral vesicle. It will be observed that the retina
is already distinctly indicated.
Fig. 379. — Later stages in development of eye (after Cardiat). a, epiblast; c, develop-
ing lens; o, optic vesicle.
so that the cavity is almost obliterated; the anterior portion,
becoming thickened, ultimately forms the retina proper, while
the posterior is represented by the tesselated pigment layer of
the choroid.
As this retinal portion breaks away from the superficial
epithelium, the latter foi'ms an elliptical mass of cells, the future
lens, the changes of which in the formation of the cells peculiar
to the lens illustrate to how great lengths differentiation in
structue is carried in the development of a single organ. It
will thus be seen that the most essential parts of the eye, the
optic nerve, the retina, and the crystalline lens, are, according
to a general law, the earliest marked out. The cornea, the iris,
the choroid, the vascular supply, the sclerotic, etc., are all sec-
ondary in importance and in formation to these, and are derived
from the mesoblast, while the essential structures are traceable,
like the nervous system itself, to the epiblastic layer.
Any act of perfect vision in a mammal may be shown to
consist of the following: (1) The focusing of rays of light from
34
530
COMPARATIVE PHYSIOLOGY.
an object on the retina, so as to form a well-defined image; (2)
the conduction of the sensory impulses thus generated in the
Fig. 380.— More advanced stage of development of eye (after Cardiat). «, epithelial
cells forming lens, now much altered; b, lens capsule; c, cutaneous tissue about
to form conjunctiva; d, e, two layers of optic vesicle, now folded back and form-
ing retina; /, mucous tissue forming vitreous humors; g, intercellular substance;
h, developing optic nerve; i, nerve-fibers entering retina.
retina by the optic nerve inward to certain centers ; and (3) the
elaboration of these data in consciousness.
We thus have the formation of an image — a physical pro-
cess; sensation, perception, and judgment— physiological and
psychical processes.
In the natural order of things we must discuss first those
arrangements which are concerned with the focusing of light —
i. e., the formation of the image on the retinal screen.
VISION.
531
DIOPTRICS OF VISION.
One of the most satisfactory methods of ascertaining that
the eye does form images of the objects in the field of vision
is to remove the eye of a recently killed albino rabbit. On
holding up before such an eye any small object, as a pair of
forceps, it may be readily observed that an inverted image of
the object is formed on the back of the eye {fundus). If,
however, the lens be removed from such an eye, no image is
formed. If the lens be itself held behind the object, an in-
verted image will be thrown upon a piece of paper held at a
suitable (its focal) distance. By substituting an ordinary bicon-
vex lens, the same effect follows. It thus appears, then, that
the lens is the essential part of the refracting media, though
the aqueous and vitreous humors and the cornea are also focus-
ing mechanisms.
In the actual human eye the focus must correspond with the
fovea of the retina if a distinct image is to be formed.
I II
Fkj. 381.— Refraction by convex lenses (after Flint and Weinhokl). The lens may be
assumed to consist of a series of lenses (II in figure), for the sake of simplicity.
though, of course, this is not strictly accurate.
It will appear that we may represent the eye as reduced to
the lens and the retina. The experiments referred to above
will convince the student that such is the case.
532 COMPARATIVE PHYSIOLOGY.
ACCOMMODATION OF THE EYE.
Using the material already referred to, the student may
observe that, with the natural eye of the albino rabbit, its
lens (or better that of a bullock's eye, being larger), or a bi-
convex lens of glass, there is only one position of the instru-
ments and objects which will produce a perfectly distinct image.
If either the eye (retina), the lens, or the object be shifted, in-
stead of a distinct image, a blurred one, or simply diffusion-
circles, appear.
A photographer must alter either the position of the object
or the position of his lens when the focus is not perfect. The
eye may be compared to a camera, and since the retina and
lens can not change position, either the shape of the lens must
change or the object assume a different position in space. As
a matter of fact, any one may observe that he can not see
objects distinctly within a certain limit of nearness to the eye,
known as the near point (punctum proximum) ■; while he be-
comes conscious of no effect referable to the eye until objects
approach within about sixty-five to seventy yards. Beyond the
latter distance objects are seen clearly without any effort.
There are many ways in which we may be led to realize
these truths : 1. When one is reading a printed page it is only
the very few words to which the eyes are then specially di-
rected that are seen clearly, the rest of the page appearing
blurred ; and the same holds for the objects in any small room.
We speak of picking out an acquaintance in an audience or
crowd, which implies that each of the individuals composing
the throng is not distinctly seen at the same time. 2. If an ob-
server hold up a finger before his eyes, and direct bis gaze into
the distance (relax his accommodation), presently he will be-
hold a second shadowy finger beside the real one — i. e., he sees
double; his eyes being accommodated for the distant objects,
can not adapt themselves at the same time for near ones.
In what does accommodation consist ? From experiments
it has been concluded that accommodation consists essentially
in an alteration of the convexity of the anterior surface of the
lens.
This change in the shape of the lens is accomplished as
follows : The lens is naturally very elastic and is kept in a par-
tially compressed condition by its capsule, to which is attached
the suspensory ligament which has a posterior attachment to
VISION.
533
the choroid and ciliary processes. When the ciliary muscle,
which operates from a fixed point the corneo-sclerotic junction,
Fig. 382. — Illustrates mechanism of accommodation (after Fick). The left side de-
picts the relation of parts during the passive condition of the eye (negative accom-
modation, or accommodation for long distances); the right side, that for near ob-
jects.
pulls upon the choroid, etc., it relaxes the suspensory ligament;
hence the lens, not being pressed upon in front as it is from
behind by the vitreous humor (invested by its hyaloid mem-
brane), is free to bulge and so increase its refractive power.
The nearer an object approaches the eye, the greater the diver-
gence of the rays of light proceeding from it, and hence the
necessity for greater focusing power in the lens.
If an animal be observed closely when looking from a remote
to a near object, it may be noticed that the eyes turn inward —
i.e., the visual axes converge and the pupils contract. These
are not, however, essential in the sense in which the changes
in the lens are ; for, as before stated, in the absence of the lens
distinct vision is quite impossible.
ALTERATIONS IN THE SIZE OF THE PUPIL.
The pupil varies in size according as the iris is in a greater
or less degree active. All observers are agreed that the circu-
lar fibers around the pupillary margin are muscular, forming
the so-called sphincter of the iris; but great differences of opin-
ion still exist in regard to the radiating fibers. It is thought
by many that all the changes in the iris may be explained by
the elasticity of its structure without assuming the existence
of muscular fibers other than those of the sphincter ; thus a
contraction of the latter would result in diminution of the pu-
53J:
COMPARATIVE PHYSIOLOGY.
pillary aperture, its relaxation to an enlargement, provided the
rest of the iris were highly elastic.
The conclusions in regard to the innervation of the iris rest
largely upon the results of certain experiments which we shall
Brain above medulla
Optic centre
Retina
Sympathetic nerve to
radiating fibres
Spinal dilator centre —
Fig. 383. — Diagram to illustrate innervation of the iris. Dotted lines indicate general
functional connection (correlation). Course of impulses indicated by arrows.
now hriefly detail : 1. When the third nerve is divided, stimu-
lation of the optic nerve (or retina) does not cause contraction
of the pupil as usual. 2. When the optic nerve is divided, light
no longer causes a contraction of the pupil, though stimulation
of the third nerve or its center in the anterior portion of the
floor of the aqueduct of Sylvius does hring about this result.
3. Section of the cervical sympathetic is followed by contrac-
VISION. 535
tion and stimulation of its peripheral end by dilatation of the
pupil.
From such experiments it has been concluded that — 1. The
optic is the afferent nerve and the third nerve the efferent nerve
concerned in the contraction of the pupil ; and that the center
in the brain is situated as indicated above, so that the act is or-
dinarily a reflex. 2. That the cervical sympathetic is the path
of the efferent impulses regulating the action of the radiating
fibers of the iris.
Its center has been located near that for the contraction of
the pupil, and it may be assumed to exert a tonic action over
the iris comparable to that of the vaso-motor center over the
blood-vessels.
The impulses may be traced through the cervical sympa-
thetic and its ganglia back to the first thoracic ganglion, and
thence to the spinal cord and brain. There may be subsidiary
centers in the cervical spinal cord.
It is to be remembered that, although the dilating center is
automatic in action, it may also act reflexly, or be modified by
unusual afferent impulses — as, e. g., the strong stimulation of
any sensory nerve which causes enlargement of the pupil
through inhibition of the center. To render the paths of
impulses affecting the iris somewhat clearer, it is well to bear
in mind the nervous supply of the part : 1. The third nerve,
through the ciliary (ophthalmic, lenticular) ganglion, supplies
short ciliary nerves to the iris, ciliary muscle, and choroid. 2.
The cervical sympathetic reaches the iris chiefly through the
long ciliary nerves and the ophthalmic division of the fifth.
3. There are sensory fibers from the fifth nerve ; and. according
to some observers, also dilating Abel's from this nerve inde-
pendent of the sympathetic, as well as those that may reach
the eye by the long ciliary nerves without entering the ciliarv
ganglion. 4. The centers from which both the contracting and
dilating impulses proceed are situated near to each other in
the floor of the aqueduct of Sylvius. It is of practical im-
portance to remember the various circumstances under which
the pupil contracts and dilates.
Contraction (Myosis).—l. Access of strong light to the
retina. 2. Associated contraction on accommodation for near
objects. 3. Similar associated contraction when the visual axes
converge, as in accommodation for near objects. 4. Reflex
stimulation of afferent nerves, as the nasal or ophthalmic divis-
536 COMPARATIVE PHYSIOLOGY.
ion of the fifth nerve. 5. During sleep. 6. Upon stimulation
of the optic or the third nerve, and the corpora quadrigemina
or adjacent parts of the brain. 7. Under the effects of certain
drugs, as physostignhn, morphia, etc.
Dilatation {Mydriasis). — 1. In darkness. 2. On stimulation
of the cervical sympathetic. 3. During asphyxia or dyspnoea.
4. By painful sensations from irritation of peripheral parts.
5. From the action of certain drugs, as atropin, etc. The
student may impress most of these facts upon his mil id by
making the necessary observations, which can be readily done.
Pathological. — As showing the importance of such connec-
tions, we may instance the fact that, in certain forms of nervous
disease (e. g., locomotor ataxia), the pupil contracts when the
eye is accommodated to near objects, but not to light (the
Argyll-Robertson pupil). In other cases, owing to brain-dis-
ease, the pupils may be constantly dilated or the reverse ; or
one may be dilated and the other contracted.
Comparative. — The iris varies in color in different groups of
animals, and even in individuals of the same group ; while the
color in early life is not always the permanent one.
In shape the pupil is elliptical in solipeds and most rumi-
nants. In the pig and dog it is circular, as also in the cat
when dilated ; but when greatly contracted in the latter animal,
it may become a mere perpendicular slit.
The iris is covered posteriorly with a layer of pigment (uvea),
portions of which may project through the pupil into the an-
terior chamber, and constitute the " sootballs " (corpora nigra)
well seen in horses, and very suggestive of inflammatory
growths, though, of course, perfectly normal.
OPTICAL IMPERFECTIONS OF THE EYE.
Anomalies of Refraction.— 1. We may speak of an eye in
which the refractive power is such that, under the limitations
referred to before (page 531), images are focused on the retina,
as the emmetropic eye. The latter is illustrated by Fig. 384.
In the upper figure, in which the eye is represented as passive
(negatively accommodated), parallel rays— i. e., rays from ob-
jects distant more than about seventy yards (according to some
writers much less) — are focused on the retina ; but those from
objects near at hand, the rays from which are divergent, are
focused behind the retina. In the lower figure the lens is rep-
VISION.
537
resented as more bulging, from accommodation, as such diver-
gent rays are properly focused.
2. In the myopic (near-sighted) eye the parallel rays cross
within the vitreous humor, and diffusion-circles being formed
on the retina, the image of the object is necessarily blurred,
Vs^r.
Fig. 384. — Diagrams to illustrate conditions of refraction in normal eye when unac-
commodated (passive, or nearly accommodated), and when accommodated for
"near" objects (after Landois).
so that an object must, in the case of such an eye, be brought
unusually near, in order to be seen distinctly — i. e., the near
■point is abnormally near and the far point also, for parallel
Fig. 385.— Anomalies of refraction in a myopic eye (after Landois).
rays can not be focused ; so that objects must be near enough
for the rays from them that enter the eye to be divergent.
The myopic eye is usually a long eye, and, though the
mechanism of accommodation may be normal, it is not so
usually, the ciliary muscle being frequently defective in some
of its fibers, which may be either hypertrophied or atrophied, or
with some affected one way and others in the opposite. More-
538 COMPARATIVE PHYSIOLOGY.
over, there is also generally, in bad cases, " spasm of accommo-
dation" (i. e., of the ciliary muscle), with increased ocular ten-
sion, etc. The remedies are, rest of the accommodation mechan-
ism and the use of concave glasses.
3. The opposite defect is hypermetropia. The hypermetropic
eye (Fig. 386), being too short, parallel rays are focused be-
hind the retina ; hence no distinct image of distant objects can
Fig. 386.— Anomalies of refraction in the hypermetropic eye (after Landois).
be formed, and they can only be seen clearly by the use of con-
vex glasses, except by the strongest efforts at accommodation.
When the eye is passive, no objects are seen distinctly beyond
a certain distance — i.e., the near point is abnormally distant
(eight to eighty inches). The defect is to be remedied by the
use of convex glasses.
4. Presbyopia, resulting from the presbyopic eye of the old, is
owing to defective focusing power, partly from diminished
elasticity (and hence flattening) of the lens, but chiefly, proba-
bly, to weakness of the ciliary muscle, so that the changes
required in the shape of the lens, that near objects may be dis-
tinctly seen, can not be made. The obvious remedy is to aid
the weakened refractive power by convex glasses. It is practi-
cally important to bear in mind that, as soon as any of these
defects in refractive power (though the same remark applies to
all ocular abnormalities) are recognized, the remedy should be
at once applied, otherwise complications that may be to a large
extent irremediable may ensue.
VISUAL SENSATIONS.
We have thus far considered merely what takes place in the
eye itself or the physical causes of vision, without reference to
those nervous changes which are essential to the perception of
VISION.
539
an object. It is true that an image of the object is formed on
the retina, but it would be a very crude conception of nervous
processes, indeed, to assume that anything resembling that
image were formed on the cells of the brain, not to speak of
the superposition of images inconsistent with that clear mem-
ory of objects we retain. Before an object is " seen," not only
must there be a clear image formed on the retina, but impulses
generated in that nerve expansion must be conducted to the
brain, and rouse in certain cells there peculiar molecular condi-
tions, upon which the perception finally depends.
For the sake of clearness, we may speak of the changes
Fig. 387. Fig. 388.
Fig. 387.— Vertical section of retina (after H. Mflller). 1, layer of rods and cones; 2
rods; 8, cones; 4, 5, 6, external granule layer; 7, interna! granule layer; !), 10 fine-
ly granular gray layer; 11, layer of nerve-cells; 1:2,14, fibers of optic nerve; 13
membrana hmitans.
Fio. 388.— Connection of rods and cones of retina with nervous elements (after Sap-
|>ey) 1,2,3, rods and cones seen from in front; 4, 5,0, side view. The rest will
be clear from the preceding figure.
540
COMPARATIVE PHYSIOLOGY.
effected in the retina as sensory impressions or impulses, which,
when completed by corresponding changes in the brain, develop
into sensations, which are represented psychically by percep-
tions ; hence, though all these have a natural connection, they
may for the moment be considered separately. It is as yet
beyond our power to explain how they are related to each
other except in the most general way, and the manner in which
a mental perception grows out of a physical alteration in the
molecules of the brain is at present entirely beyond human
comprehension .
Affections of the Eetina.— There is no doubt that the fibers of
the optic nerves can not of themselves be directly affected by
light. This may be experimentally demonstrated to one's self
by a variety of methods, of which the following is readily car-
ried out : Look at the circle (Fig. 389) on the left hand with the
Fig. 389 (after Bernstein).
right eye, the left being closed, and, with the page about twelve
to fifteen inches distant, gradually approximate it to the eye,
when suddenly the cross will disappear, its image at that dis-
j:<)j^ry:;:t;:l;r,!r';''^. ...;■■'.■'../■;, "":i" ...j, ■•■■.,■':■■' ■■';:i,',\'ui/il,,!,";,/lj,i!1,
m/m
S-e-
Fig. 390.— Diagrammatic section of macula lutea in man (after Huxley), a, a, pigment
of choroid; b, c, b, c, rods and cones; d, d, outer granular or nuclear layer; /,/,
inner granular layer; g, ij, molecular layer; h, h, layer of nerve-cells; i, i, fibers of
optic nerve.
VISION. 511
tance having fallen on the blind-spot, or the point of entrance
of the optic nerve.
It remains, then, to determine what part of the retina is
affected by light. The evidence that it is the layers of rods and
cones is convincing. It has been shown that parts of the retina
itself internal to these layers cast perceptible, shadows, the con-
clusion that the rods and cones are the essential parts of the
sensory organ would be inevitable.
The Laws of Retinal Stimulation. — It may be noticed that,
when a circular saw in a mill is rotated with extreme rapidity,
it seems to be at rest.
If a stick on fire at one end be rapidly moved about, there
seems to be a continuous fiery circle.
If a top painted in sections with various colors be spun, the
different colors can not be distinguished, but there is a color
resulting from the blending of the sensations from them all,
which will be white if the spectral colors be employed.
When, on a dark night, a moving animal is illuminated by
a flash of lightning, it seems to be at rest, though the attitude is
one we know to be appropriate for it during locomotion.
It becomes necessary to explain these and similar phe-
nomena. Another observation or two will furnish the data for
the solution.
If on awakening in the morning, when the eyes have been
well rested and the retina is therefore not so readily fatigued,
one looks at the window for a few seconds and then closes the
eyes, he may perceive that the picture still remains visible as
a positive after-image ; while, if a light be gazed upon at night
and the eyes suddenly closed, an after-image of the light may
be observed.
It thus appears, then, that the impression or sensation out-
lasts the stimulus in these cases, and this is the explanation
into which all the above-mentioned facts fit. When the fiery
point passing before the eyes hi the case of the fire-brand stimu-
lates the same parts of the retina more frequently than is con-
sistent with the time required for the previous impression to
fade, there is, of necessity, a continuous sensation, which is in-
terpreted by the mind as referable to one object. In like man-
ner, in the case of a moving object seen by an electric flash, the
duration of the latter is so brief that the object illuminated can
not make any appreciable change of position while it lasts; a
second flash would show au alteration, another part of the
542 COMPARATIVE PHYSIOLOGY.
retina being stimulated, or the original impression having
faded, etc.
In the case of a top or (better seen) color-disk, painted into
black and white sectors, it may be observed that with a faint
light the different colors cease to appear distinct with a slower
rotation than when a bright light is used. The variation is
between about TV and -5V of a second, according to the intensity
of the light used. Fusion is also readier with some colors than
others.
It is a remarkable fact that one can distinguish as readily
between the quantity of light emanating from 10 and 11 candles
as between 100 and 110.
The Visual Angle. — If two points be marked out with ink on
a sheet of white paper, so close together that they can be just
distinguished as two at the distance of 12 to 20 inches, then on
removing them a little farther away they seem to merge into
one.
The principle involved may be stated thus : When the dis-
tance between two points is such that they subtend a less visual
angle than 60 seconds, they cease to be distinguished as two.
Fig. 391 illustrates the visual angle. It will be noticed that a
larger object at a greater distance subtends the same visual
angle as a smaller one much nearer. The size of the retinal
Fig. 391.— The visual angle. The object at A" appears no larger than the one at .4
(Le Conte).
image corresponding to 60 seconds is "004 mm. (4 n), and this
is about the diameter of a single rod or cone. It is not, how-
ever, true that when two cones are stimulated two objects are
inferred to exist in every case by the mind ; for the retina va-
ries in different parts very greatly in general sensibility and in
sensibility to color.
It is noticeable that visual discriminative power can be
greatly improved by culture, a remark which applies especially
to colors. It seems altogether probable that the change is cen-
tral in the nerve-cells of the part or parts of the brain con-
VISION. 543
cerned, especially of the cortical region, where the cell processes
involved in vision are finally completed.
Color- Vision, — As we are aware by experience the range and
accuracy of color perception in man is very great, though vari-
able for different persons, a good deal being dependent on culti-
vation. However, there are also pronounced natural differ-
ences, some individuals being unable to differentiate between
certain primary colors as red and green, and so are " color-
blind." It is of course difficult to determine in how far the
lower animals can discriminate between colors ; but in certain
groups, as the birds, it would seem to be reasonable to conclude
that their color-perceptions are highly developed.
It is further probable that in this group, and possibly some
others with the eyes placed more in the lateral than the anterior
portion of the head, the retinal area for the most distinct vision,
including that for colors is larger than in man, at all events.
PSYCHOLOGICAL ASPECTS OF VISION.
It is impossible to ignore entirely, in treating of the physi-
ology of the senses, the mind, or perceiving ego.
By virtue of our mental constitution, we refer what we " see "
to the external world, though it is plain that all that we per-
ceive is made up of certain sensations.
We recognize the " visual field " as that part of the outer
world within which alone our vision can act at any one time ;
and this is, of course, smaller for one than for both eyes.
If one takes a large sheet of paper and marks on its center
a spot on which one or both eyes are fixed, by moving a point
up or down, to the right or the left, he may ascertain the limits
of the visual field for a plane surface. The visual field for both
eyes measures about 180° in the horizontal meridian ; for one
eye about 145°; and in the vertical meridian 100°.
After-images, etc. — Positive after-images have already been
referred to ; but an entirely different result, owing to exhaus-
tion of the retina, may follow when the eye is turned from the
object. If, after gazing some seconds at the sun, one turns
away or merely closes the eyes, he may see black suns. In
like manner, when one turns to a gray surface after keeping
the eyes fixed on a black spot on a white ground, he will see a
light spot. Such are termed negative after-images, and these
may themselves be colored, as when one turns from a red to a
544
COMPARATIVE PHYSIOLOGY.
white surface and sees the latter green,
siclered as the results of exhaustion.
They may be con-
CO-ORBINATION OF THE TWO EYES IN VISION.
As a matter of fact, we are aware that an object may be
seen as one either with a single eye or with both. For bi-
nocular vision it may be shown that
the images formed on the two retinas
must fall invariably on corresponding
points.
The position of the latter may be
gathered from Fig. 392. It will be no-
ticed that the malar side of one eye
corresponds to the nasal side of the
other, though upper always answers to
upper and lower to lower. This may
also be made evident if two saucers
(representing the fundus of each eye)
be laid over each other and marked off,
as in the figure.
That such corresponding points do
Fig. 392.— Diagram to illus- actually exist maybe shown by turniug
(afterC Foster)11 z?j?°leit one eye so that the image shall not
pltttteo?,ee8;eye''iree- &11, *> indicated in the figure. Only
eponding to ax, bu cx, m now au(j then, however, is a person to
the other. The lower fig- ' A
ures are projections of the be found wTho can voluntarily acco.n-
tffitaft (l?ey"S Itmay be plish this, but it occurs in all kinds of
side'oTone'reti^ fovrl natural or induced squint, as inalcohol-
■ sponds to the nasal side of {sm owing1 to partial paralysis of some
tliG other —Z.
of the ocular muscles. We are thus
naturally led to consider the action of these muscles.
Ocular Movements. — Upon observing the movements of an
individual's eyes, the head being kept stationary, it may be
noticed that (1) both eyes may converge ; (2) one diverge and
the other turn inward ; (3) both move upward or downward ;
(4) these movements may be accompanied by a certain degree
of rotation of the eyeball.
The eye can not be rotated around a horizontal axis without
combining this movement with others. To accomplish the
above movements it is obvious that certain muscles of the six
with which the eye is provided must work in harmony, both as
VISION.
545
to the direction and degree of the movement — i. e., the move-
ments of the eyes are affected hy very nice muscular co-ordina-
tions.
Fig. 303.— View of the two eyes and related parts (after Helmholtz.)
Fig-. 394 is meant to illustrate diagrammatically the move-
ments of the eyeball.
While the several recti muscles elevate or depress the eye,
and turn it inward or outward, and the oblique muscles rotate
it, the movements produced by the superior and inferior recti
always corrected by the assistance of the oblique muscles, since
the former tend of themselves to turn the eye somewhat in-
ward. In like manner the oblique muscles are corrected by
the recti. The following tabular statement will expi*ess the
conditions of muscular contraction for the various movements
of the eye in man :
Straight
move-
ments;.
Elevation Rectus superior and obliquus in-
ferior.
Depression Rectus inferior and obliquus su-
perior.
| Adduction to nasal side. . .Rectus interims,
i. Adduction to malar side. . . Rectus externus.
35
546
COMPARATIVE PHYSIOLOGY.
Oblique
move- -
ments.
r Elevation with adduction. . Rectus superior and internus,
with obliquus inferior.
Depression with adduction.Reetus inferior and internus,
with obliquus superior.
Elevation with abduction. . Rectus superior and externus,
with obliquus inferior.
Depression with abduction.Rectus inferior and externus,
with obliquus superior.
"What is the nervous mechanism by which these " associ-
ated " movements of the eyes are accomplished ? It has been
found, experimental-
ly, that when different
parts of the corpo-
ra quaclrigemina are
stimulated, certain
movements of»the eyes
follow. Thus stimu-
lation of the right side
of the nates leads to
movements of both
eyes to the left, and
the reverse when the
opposite side is stimu-
lated ; also, stimula-
tion in the middle line
causes convergence
and downward move-
ment, etc., with the
corresponding move-
ments of the iris.
Since section of the
nates in the middle
line leads to move-
ments confined to the
eye of the same side,
the center would ap-
pear to be double.
However, it may be that the cells actually concerned do not lie
in the corpora quadrigemina, but below or outside of them. The
localization is as yet incomplete. In many groups of animals,
including the solipeds, ruminants, and Carnivoi^a, there is a
posterior rectus or retractor oculi by which the eye may be
Fig. 304.— Diagram intended to illustrate action of
extrinsic ocular muscles (after Pick). The heavy
lines represent the muscles of the eyeball, and the
fine lines the axes of movement.
VISION.
547
Fig. 395. — Diagrammatic section of the eye of the horse (Chauvean). a, optic nerve;
b. sclerotic coat; c, choroid; d, retina; e, cornea; /, iris; g, h, ciliary ligament and
processes of choroid represented as separated from it, the better to define its lim-
its; i, insertion of ciliary processes on crystalline lens; j, crystalline lens; k\ lens
capsule; /, vitreous body; m,m, anterior and posterior chambers; o. theoretical
indication of aqueous humor; p, p. tarsi (eyelids); q, q, fibrous membrane of eye-
lids; r, elevator muscle of upper eyelid; s. s, orbicularis muscle of lids; t, t, skin
of eyelids; u, conjunctiva; ?>, epidermic layer of the latter covering cornea; x,
posterior rectus muscle; y, superior rectus; z, inferior rectus; w, fibrous sheath of
orbit (orbital membrane).
drawn inward and thus protected the more effectually against
blows and obstacles. It seenis to be of special importance in
animals that feed with the head
down for long- periods, as in the
ruminants, in which class it is
most highly developed.
The macula lutea is believed
to exist only in man, the quadra-
man a. and certain of the lizard
tribe — i. e., in animals in which
the axes of the eyeballs are parallel
to each other. Nevertheless, there
is no reason to doubt that the cen-
tral part of the retina is more sensitive than the periphery or
that there is a central retinal zone for distinct vision in all
vertebrates, though not so limited in all cases as in man.
Fig. 896. — Diagram to illustrate de-
cussation of fibers in the optic
commissure of man (after Flint).
548 COMPARATIVE PHYSIOLOGY.
Estimation of the Size and Distance of Objects.— The pro-
cesses by which we form a judgment of the size and distance of
objects are closely related.
As we have already shown (page 542), the visual angle varies
both with the size and the distance of an object. Knowing
that two objects are at the same distance from the eye, we esti-
mate that the one is larger than the other when the image one
forms on the retina is larger, or when the visual angle it sub-
tends is greater than in the other case, and conversely. Thus,
knowing that two persons are at the distance of half a mile
away, if one is judged by us to be smaller than the other, it
will be because the retinal image corresponding to the object
is smaller, other things being equal. But the subject is more
complex than might be inferred from these statements.
Objects of a certain color seem nearer than others ; also those
that are brighter, as in the case of mountains on a clear day.
And not only do all the qualities of the image itself enter as
data into the construction of the judgment, but numerous mus-
cular sensations. The eyes accommodating and converging for
near objects, from the law of association, give rise to the idea of
nearness, for habitually such takes place when near objects are
viewed, so that the subject becomes very complex. That we
judge imperfectly of the position of an object with but one eye
is realized on attempting to stick a pin into a certain small spot,
thread a needle, cork a small bottle, etc., when one eye is closed.
Solidity. — By the use of one eye alone we can form an idea
of the shape of a solid body ; though, in the case of such as are
very complex, this process is felt to be both laborious and im-
perfect.
From the limited nature of the visual field for distinct
vision, it follows that we can not with one eye see equally dis-
tinctly all the parts of a solid that is turned toward us. After
a little practice one may learn to define for himself what he
actually does see.
Such a figure as that following results from the combina-
tion, mentally, of two others, which answer to the images fall-
ing on the right and on the left eye respectively.
In order that such fusion shall take place, the respective
images must fall on identical (corresponding) parts of the
retina.
As is well known, the pictures used for stereoscopes give
different views of the one object, as represented on a flat sur-
VISION.
549
face. These are thrown upon corresponding points of the retina
by the use either of prisms or mirrors, when the idea of solidity
is produced. As to whether movements of the eyes (conver-
gence) are necessary for stereoscopic vision is disputed. It has
d/
a
Fig. 397. — Illustrates binocular vision. If the truncated pyramid, P, be looked at
with the head held perpendicularly over the figure, the image formed in the right
eye when the left is closed is figured on the right, and that seen when the right
eye is closed is represented by the figure in the middle. No superposition of these
figures will give P, yet by a pyschic process they are combined into P, the figure
as it appears to both eyes (after Bernstein).
been inferred, from the fact that objects appear solid during
an electric flash, the duration of which is far too short to per-
mit of movements of the ocular muscles, that such movements
are not essential. The truth seems to lie midway ; for while
simple figures may not require them, the more complex do, or,
at all events, the judgment is very greatly assisted thereby. It
is of the utmost importance to bear in mind that all visual
judgments are the result of many processes, in which, not the
sense of vision alone, but others, are concerned; and the mutual
dependence of one sense on another is great, probably beyond
our powers to estimate. Reference has been made to this sub-
ject previously.
PROTECTIVE MECHANISMS OF THE EYE.
The eyelids have been appropriately compared to the shut-
ters of a window. They are, however, not impervious to light,
as any one may convince himself by noticing that he can locate
the position of a bright light with the eyes shut; also that a
sensitive person (child) will turn away (reflexly) from a light
when sleeping if it be suddenly brought near the head. The
Meibomian glands, a modification of the sebaceous, secrete an
oily substance that seems to protect the lids against the lachry-
mal fluid, and prevents the latter running over their edges as
oil would on the margins of a vessel. The lachrymal gland is
550
COMPARATIVE PHYSIOLOGY.
not in structure unlike the parotid, the secretion of which its
own somewhat resembles.
The saltness of the tears, owing to abundance of sodium
chloride, is well known to all. The nervous mechanism of se-
cretion of tears is usually reflex, the stimulus coming from the
action of the air against the eyeball or from partial desiccation
owing to evaporation. When the eyeball itself, or the nose, is
irritated, the afferent nerves are the branches of the fifth, to
which also belong the efferent nerves. Tbe latter include also
the cervical sympathetic. But it will, of course, be understood
that the afferent impulses may be derived through a large num-
ber of nerves, and that the secreting center may be acted upon
directly by the cerebrum (emotions). The excess of lachrymal
secretion is carried away by the nasal duct into which tbe lach-
rymal canals empty. While it is well known that closure of the
lids by the orbicularis muscle favors the removal of the fluid, the
method by which the latter is accomplished is not agreed upon.
Some believe that the closure of the lids forces the fluid on
through the tubes, when they suck in a fresh quantity ; others that
the orbicularis drives the fluid directly through the tubes, kept
open by muscular arrangements ; and there are several other
divergent opinions. The prevention of winking leads to irrita-
tion of the eye, which may assume a serious character, so that
the obvious use of the secretion of tears
is to keep the eye both moist and clean.
Though rudimentary in man, there
is in all our domestic animals a third
eyelid {membrana nictitans) which
may be made to sweep over the eye
and thus cleanse it. It is especially
well developed in those groups of
mammals that can not derive assist-
ance in wiping the eyes from their fore-
limbs, hence is found in perfection in
solipeds and ruminants. It is made up
of a fibro - cartilage, prismatic at its
base, and thus anteriorly where it is
covered by the conjunctiva. It is most
attached at the inner can thus of the
eye, from which region it can spread
over the whole globe anteriorly. The fibro-cartilage is con-
tinued backward by a fatty cushion which is loosely attached
Fig. 398.— Lachrymal canals,
lachrymal sac, and nasal
canal in man opened from
the front (after Sappey.).
VISION. 551
to all the ocular muscles. When the globe of the eye is with-
drawn by its muscles, the third eyelid is pushed out in a me-
chanical way with little or no direct assistance from muscles.
In this connection may also be mentioned the gland of
Harder, a yellowish red glandular structure situated about the
middle of the outer surface of the third eyelid, which furnishes
a thick unctuous secretion, also of a protective character. These
structures are all the more necessary, as in few animals is the
globe of the eye so well protected by bony walls as in man.
SPECIAL CONSIDERATIONS*
Comparative. — It seems to be established that some animals
devoid of eyes, as certain myriopods, are able to perceive the
presence of light, even when the heat-rays are cut off. The most
rudimentary beginning of a visual apparatus appears to be a
mass of pigment with a nerve attached, as in certain worms ;
though it is questionable whether mere collections of pigment
without nerves may not in some instances represent still earlier
rudiments of our eyes.
The eye of the fish is characterized by flatness of the cornea ;
spherical form of the lens, the anterior surface of which pro-
jects far beyond the pupillary opening ; the presence of a pro-
cess of the choroid {processus falciformis) ; and usually the ab-
sence of eyelids, the cornea being covered with transparent skin.
The eye of the bird, in some respects the most perfect visual
organ known, is of peculiar shape as a whole, presenting a large
posterior surface for retinal expansion ; a very convex cornea,
a highly developed lens, an extremely movable iris ; eyelids
and a nictitating membrane (third eyelid), which may be made
to cover the whole of the exposed part of the eye, and thus
shield screen -like from excess of light ; ossifications of the scle-
rotic ; a structure which is a peculiar modification of the
choroid, of which it is a sort of offshoot and like it very vascular,
answering to the falciform process of the eye of the fish and the
reptile. From its appearance it is termed the pecten. Birds, on
account of a highly developed ciliary muscle, possess wonderful
powers of accommodation, rendered important on account of
their rapid mode of progression. They also seem to be able to
alter the size of the pupil at will. Their iris is composed of
striped muscular fibers.
A layer of fibrous tissue outside of the choroidal epithelium
552
COMPARATIVE PHYSIOLOGY.
forms the tapetum. It is most pronounced in the carnivora
and gives the glare to their eyes as well seen in the cat tribe at
night. It has been supposed to act as a reflector and thus
assist in vision in the same way as a pair of carriage lamps
light up the roadway.
Evolution. — From the above brief account of the eye in dif-
ferent grades of animals, it will ap-
pear that its modifications answer to
differences in the environment.
Adaptation is evident. Darwin
believes this to have been effected
partly by natural selection — i. e., the
survival of the animal in which the
form of eye appeared best adapted to
its needs — and partly by the use or
disuse of certain parts.
The latter is illustrated by the
blind fishes, insects, etc., of certain
caves, as those of Kentucky; and it
is of extreme interest to note that
various grades of transition toward
complete blindness are observable,
CM .ciliary muscle. Birds have according to the degree of darkness
usually keen vision, great pow- , ° °
er of accommodation, and ex- in which the animal lives, whether
treme mobility of the iris. , ,, .., . ,, ,
wholly within the cave or where
there is still some light. A crab has been found with the eye-
stalk still present, but the eye itself atrophied. Again, ani-
mals that burrow seem to be in process of losing their eyes,
through inflammation from obvious causes ; and some of them,
as the moles, have the eye still existing, though well-nigh or
wholly covered with skin. Internal parasites are often with-
out eyes. It is not difficult to understand how one bird of prey,
with eyes superior to those of its fellows, would gain supremacy,
and, in periods of scarcity, survive and leave offspring when
others would perish.
It is, of course, impossible to trace each step by which the
vertebrate eye has been developed from more rudimentary
forms, though the data for such an attempt have greatly accu-
mulated within the last few years; and it is not to be forgotten
that even the vertebrate eye has many imperfections, so that
no doctrine of complete adaptation, according to the argument
from design as usually understood, can apply.
Fig. 399.— Eye of nocturnal bird
of prey (after Wiedersheim).
Co, cornea; L, lens; Rt, retina;
P, pecten; No, optic nerve; Sc,
ossification of sclerotic coat;
VISION.
553
It is of great importance to recognize that what we really
see depends more upon the brain and the mind than the eye.
If any one will observe how frequent are his incipient errors
of vision speedily corrected, he will realize the truth of the
Brain above
medulla
Centre in region of-
corp. quadrigemina
-Cortical centre
Centre in optic thalamus
etina
Fig. 400. — Diagram intended to illustrate the elaboration of visual impulses, beginning
in retina and culminating in the cerebral cortex. Course of impulses is indicated
by arrows. Knowledge of auditory centers is not yet exact enough to permit of
the construction of a diagram, though doubtless eventually the central processes
will be localized as with vision. The latter remark applies" to the other senses to
nearly the same extent, possibly quite as much.
above remark. Precisely the same data furnished by the eye
are in one mind wox\ked up in virtue of past experience (edu-
cation) into an elaborate conception, while to another they an-
swer only to certain vague forms and colors. And herein lies
the great superiority of man's vision over that of all other
animals.
Within the limits of their mental vision do all creatures see.
Man has not the keen ocular discriminating power of the hawk;
he can neither see so far nor so clearly ; nor has he the wide
field of vision of the gazelle; but he has the mental resource
which enables him to make more out of the materials with
which his eyes furnish him. It is by virtue of his higher cere-
bral development that he has added to his natural eyes others
554 COMPARATIVE PHYSIOLOGY.
in the microscope and telescope, which none of Nature's forms
can approach.
Pathological. — There may be ulceration of the cornea, in-
flammation of this part, or various other disorders which lead
to opacity. The low vitality of this region, probably owing to
absence of blood-vessels, is evidenced by the slowness with
which small ulcers heal. Opacity of the lens (cataract) when
complete causes blindness, which can be only partially reme-
died by removal of the former. Inflammations of any part of
the eye are serious, from possible adhesions, opacities, etc., fol-
lowing. Should such be accompanied by great excess of intra-
ocular tension, serious damage to the retina may result. Of
course, atrophy of the optic nerve (due to lesions in the brain,
etc.) is irremediable, and involves blindness. Inspection of the
internal parts of the eye (fundus oculi) often reveals the first
evidence of disease in remote parts as the kidneys.
From what has been said of the movements of the two eyes
in harmony, etc. , the student might be led to infer that disease
of one organ, in consequence of an evident close connection of
the nervous mechanism of the eyes, would be likely to set up
a corresponding condition in the other unless speedily checked.
Such is the case, and is at once instructive and of great practi-
cal moment.
Paralysis of the various ocular muscles leads to squinting,
as already noticed.
Brief Synopsis of the Physiology of Vision.— All the other
parts of the eye may be said to exist for the retina, since all are
.related to the formation of a distinct image on this nervous ex-
pansion. The principal refractive body is the crystalline lens.
The iris serves to regulate the quantity of light admitted to the
eye, and to cut off too divergent rays. In order that objects at
different distances may be seen distinctly, the lens alters in
shape in response to the actions of the ciliary muscle on the
suspensory ligament, the anterior surface becoming more con-
vex. Accommodation is associated with convergence of the
visual axes and contraction of the pupil. The latter has circular
and radiating plain muscular fibers (striped in birds, that seem
to be able to alter the size of the pupil at will), governed by the
third, fifth, and sympathetic nerves. Contraction of the pupil
is a reflex act, the nervous center lying in the front part of the
Hoor of the aqueduct of Sylvius, while the action of the other
center (near this one) through the sympathetic nerve is tonic.
VISION. 555
Accommodation through the ciliary muscle is governed hy
a center situated in the hind part of the floor of the third ven-
tricle near the anterior bundles of the third nerve, which latter
is the medium of the change. When rays of light are focused
anterior to the retina, the eye is myopic ; when posterior to it,
hypermetropic.
The presbyopic eye is one in which the mechanism of accom-
modation is at fault, chiefly the ciliary muscle. The point of
entrance of the optic nerve (blind-spot) is insensible to light;
and visual impulses can be shown to originate in the layers of
rods and cones, probably through stimulation from chemical
changes effected by light acting on the retina. The sensation
outlasts the stimulus ; hence positive after-images occur. Nega-
tive after-images occur in consequence of excessive stimulation
and exhaustion of the retina, or disorder of the chemical pro-
cesses that excite visual impulses. When stimuli succeed one
another with a certain degree of rapidity, sensation is continu-
ous. The eye can distinguish degrees of light within certain
limits, varying by about -j^-g- of the total.
Objects become fused or are seen as one when the rays
from them falling on the retina approximate too closely on
that surface. The brain, as well as the eye itself, is concerned
in such discriminations, the former probably more than the
latter.
The macula lutea, and especially the fovea centralis, are in
man the points of greatest retinal sensitiveness. When the
images of objects are thrown on these parts, they are seen with
complete distinctness; and it is to effect this result that the
movements of the two eyes in concert take place. An object is
seen as one when the position of the eyes (visual axes) is such
that the images formed fall on corresponding parts of the retina.
Binocular vision is necessary to supply the sensory data for the
idea of solidity. It is important to remember that, before an .ob-
ject is " seen " at all, the sensory impressions furnished by the
retina and conveyed inward by the optic nerve are elaborated in
the brain and brought under the cognizance of the perceiving
ego. We recognize many visual illusions and imperfections
of various kinds, the course of which it is difficult to locate
in any one part of the visual tract, such as are referred to
" irradiation," " contrast," etc. Tbere may also be visual phe-
nomena that are purely subjective, and others that result from
suggestion rather than any definite sensory basis of retinal
556 COMPARATIVE PHYSIOLOGY.
images. Hence what one sees depends on his state of mind at
the time.
This applies to appreciation of size and distance also, though
in such cases we have the visual angle, certain muscular move-
ments (muscular sense), the strain of accommodation etc., as
guides.
HEARING.
As the end organ oi vision is protected both without and
within, so is the still more complicated end-organ of the sense
of hearing more perfectly guarded against injury, being in-
closed within a membranous as well as bony covering and sur-
rounded by fluid, which must shield it from stimulation, except
through this medium.
Hearing proper, as distinguished from the mere recognition
of jars to the tissues, can, in fact, only be attained through the
impulses conveyed to the auditory brain-centers, as originated
in the end-organ by the vibrations of the fluid with which it is
bathed.
It will be assumed that the student has made himself famil-
iar with the general anatomy of the ear. The essential points
in regard to sound are considered in the chapter on The
Voice. It will be remembered that what we term a musical
tone, as distinguished from a noise, is characterized by the
regularity of vibrations of the air that reach the ear ; and that
just as ethereal vibrations of a certain wave-length give rise to
the sensation of a particular color, so do aerial vibrations of a
definite wave-length originate a certain tone. In each case
must we take into account a physical cause for the physiological
effect, and these bear a very exact relationship to one another.
As will be seen later, while in all animals that have a well-
defined sense of hearing the process is essentially such as we
have indicated above, the means leading up to the final stimu-
lation of the end-organ are very various. At present we shall
consider the acoustic mechanism in mammals, with special ref-
erence to man. There are in fact three sets of apparatus : (1)
one for collecting the aerial vibrations; (2) one for transmit-
ting them ; and (3) one for receiving the impression through a
fluid medium; in other words, an external, middle, and in-
ternal ear.
558
COMPARATIVE PHYSIOLOGY.
The external ear in man being- practically immovable, owing
to the feeble development of its muscles, has, as compared with
;:ittl-n
Ftg. 401. —Section through auditory organ (after Sappey). 1, pinna; 2, 4, 5, cavity of
concha, external and auditory meatus with opening of ceruminous glands; 6,
membrana tympani; 7. anterior part of incus; 8, malleus; 9, long handle of mal-
leus, attached to internal surface of tympanic membrane— it is here represented as
strongly indrawn; 10, tensor tympani muscle; 11, tympanic cavity; 12, Eustachian
tube; 13, superior semicircular canal; 14, posterior semicircular canal; 15, exter-
nal semicircular canal; 16, cochlea; 17, internal auditory meatus; 18, facial nerve;
19. large petrosal nerve; 20, vestibular branch of auditory nerve; 21, cochlear
branch of same.
such animals as the horse or cow, but little use as a collecting'
organ for the vibrations of the air. The meatus or auditory
canal may be regarded both as a conductor of vibrations and
as protective to the middle ear, especially the delicate drum-
head, since it is provided with hairs externally in particular,
and with glands that secrete a bitter substance of an unctuous
nature.
The Membrana Tympani is concavo-convex in form, and
having attached to it the chain of bones shortly to be noticed,
is well adapted to take up the vibrations communicated to it
from the air; though it also enters into sympathetic vibration
when the bones of the head are the medium, as when a tuning-
fork is held between the teeth. Ordinary stretched membranes
have a fundamental (self-tone, proper tone) tone of their own,
to which they respond more readily than to others.
HEARING. 559
If such held for the membrana tympani, it is evident that
certain tones would be heard better than others, and that when
Fig. 402.— Photographic representation of right membrana tympani, viewed from
within (after Flint and Uiidinger). 1. divided head of malleus; 2. neck; 3. handle,
with attachment of tendon of tensor tympani; 4, divided tendon; 5. 6, long handle
of malleus; 7, outer radiating and inner circular fibers of tympanic membrane; 8,
fibrous ring encircling membrana tympani; 9, 14, 15, dentated fibers of Gruber; 10,
11. posterior pocket connecting with malleus; 12, anterior pocket; 13, chorda tym-
pani nerve.
the fundamental one was produced the result might be a sensa-
tion unpleasant from its intensity. This is partially obviated
by the damping- effect of the auditory ossicles, which also pre-
vent after-vibrations.
Some suppose that what we denominate shrill or harsh
sounds are, in part at least, owing- to the auditory meatus hav-
ing a corresponding fundamental note of its own.
The Auditory Ossicles.— Though these small bones are con-
nected by perfect joints, permitting a certain amount of play
560
COMPARATIVE PHYSIOLOGY.
upon one another, experiment has shown that they vibrate in
response to the movements of the drum-head en masse ; though
the stapes has by no means the range of movement of the han-
dle of the malleus ; in other words, there is loss in amplitude,
Fig. 403.— Section of auditory organ of horse (after Chauveau). A, auditory canal;
B, membrana tympani; C, malleus; D, incus; F, stapes; O, mastoid cells; //,
fenestra ovalis; I, vestibule; ./, K, L, outline of semicircular canals; M, cochlea;
N, commencement of scala tympani.
but gain in intensity. A glance at Fig. 404 will show that the
end attained by this arrangement of membrane and bony levers,
which may be virtually reduced to one (as it is in the frog, etc.),
is the transmission of the vibrations to the membrane of the
fenestra ovalis, through the stapes finally, and so to the fluids
within the internal car. But it might be supposed that, for the
avoidance of shocks and the better adaptation of the apparatus
HEARING.
561
to its work, some regulative apparatus, in the form of a nerv-
ous and muscular mechanism, would have been evolved in the
Fig. 404.— Diagrammatic representation illustrating auditory processes (after Beaunis).
A, external ear; B, middle ear: C, internal ear; 1, auricle; 2. external auditory
meatus; 3, tympanum; 4, membrana tympani; 5, Eustachian tube; 6. mastoid
cells; 10, foramen rotundum; 11, foramen ovale; 12. vestibule; 13, cochlea; 14,
scala tympani; 15, scala vestibuli; 16, semicircular canals.
N. B. — The ear is so complicated an organ that it is almost impossible to give a dia-
grammatic representation of it at once simple and complete in a single figure.
A comparison of the whole series of cuts is therefore desirable. It is essential to
understand how the end-organ within the scala media is stimulated.
higher groups of animals. Such is found in the tensor tym-
pani, laxator tympani, and stapedius muscles, as well as the
Eustachian tube.
Muscles of the Middle Ear. — The tensor tympani regulates
the degree of tension of the drum-head, and hence its ampli-
tude of vibration, having a damping effect, and thus preventing
the ill results of very loud sounds.
Ordinarily, this is. doubtless, a reflex act, in which the fifth
is usually the afferent nerve concerned. It is well-known that,
when we are aware that an explosion is about to take place, we
are not as much affected by it, which would seem to argue a
voluntary power of accommodation ; but of this we must speak
with caution.
According to some authorities the laxator tympani is not a
36
562
COMPARATIVE PHYSIOLOGY.
muscle, but a supporting ligament for the malleus. The stape-
dius, however, has the important function of regulating the
movements of the stapes, so that it shall not be too violently
driven against the membrane covering the fenestra ovalis.
The two muscles, stapedius and tensor, suggest the accom-
modative mechanism of the iris. The motor nerve of the sta-
pedius is derived from the facial; of the tensor, from the tri-
geminus through the otic ganglion.
The Eustachian Tube. — Manifestly, if the middle ear were
closed permanently, its air would gradually be absorbed. The
drum-head would be thrust in by atmospheric pressure, and
become useless for its vibrating function. The Eustachian
tube, by communicating with the throat, keeps the external and
internal pressure of the middle ear balanced. Whether this
canal is permanently open, or only during swallowing, is as yet
undetermined.
■■:-:-^:^%-":' ■>-..
PlG. 405.— Diagram intended to illustrate the processes of hearing (after Landois) A G,
external auditory meatus; 7\ tympanic membrane: A", malleus; a, incus; P, mid-
dle ear; o, fenestra ovalis; r, fenestra rotunda; pi, seala tympani; vt, scala vesti-
Imli; I'. vestibule; 8, saccnle; U, utricle; H, semicircular canals; TE, Eustachian
tube. Long arrow indicates line of traction of tensor tympani; short curved one
that of Stapedius.
One may satisfy himself that the middle ear and pharynx
communicate, by closing the nostrils and then distending the
upper air-passages by a forced expiratory effort, when a sense
of distention within the ears is experienced, owing to the rise
of atmospheric pressure in the tympanum.
HEARING,
563
Fig. 406. — Section through one of the coils of cochlea (after Chauveau). ST. scala tym-
pani; SV, scaja vestibuli; CC, cochlear canal (scala media); Co, organ of Corti:
11, membrane of Reissner; b, membrana basilaris; feo, lamina spiralis ossea: I,
membrana tectoria; 1, 2, rods of Corti; nc, cochlear nerve with its ganglion, gs.
Pathological. — Inflammation of the tympanum may result
in adhesions of the small bones to other parts or to each other,
Fid. 407.— I. Transverse section of a turn of cochlea. TI. Ampulla of a semicircular
canal and its crista acoustica; ap, auditory cells, one of which is a hair-cell. 111.
Diagram of labyrinth of man. IV. Of bird. V. Of fish. (After Landois.i
564
COMPARATIVE PHYSIOLOGY.
Fig. 408.— Diagrammatic representation of ductus cochlearis and organs of Corti (after
Landois). N, nerve of cochlea;- A', inner, and P, outer, hair-cells; n, nerve-fibrils
terminating in P; a, a, supporting cells; d, cells of sulcus spiralis; z. inner rod
of Corti; y, outer rod of Corti; mb, membrane of Corti (membrana tectoria); o,
membrana reticularis; H, G, cells of area toward outer wall.
A.N.
Coch.
Fig. 410.
Fio. 409.
Fig. 409. — Auditory epithelium from macula acoustica of saccule of alligator, much
magnified (after Schafer). c. c. columnar hair-cells; f,f, fiber cells; n, nerve-fiber
losing its medullary sheath and about to terminate in "columnar auditory cells; h,
auditory hair; h' , base of auditory hairs split up into fibrils.
I'ii;. 410. — Diagrammatic representation of distribution of auditory nerve in membra-
nous labyrinth and cochlea (after Huxley).
HEARING.
565
•or to occlusion of the Eustachian tube from excess of secretion,
cicatrices, etc., in consequence of which the relations of atmos-
pheric pressure become altered, the membrana tympani being
indrawn, and the whole series of conditions on which the nor-
mal transmission of vibrations depends disturbed, with the
natural result, partial deafness. The hardness of hearing- ex-
perienced during a severe cold in the head (catarrh, etc.) is
owing in great part to the occlusion of the Eustachian tube,
which may be either partial or complete.
By filling one or both of the ears external to the mem-
brana tympani with cotton-wool, one may satisfy himself how
essential for hearing is the vibratory mechanism, which is, of
course, under such circumstances inactive or nearly so ; hence
the deafness.
When the middle ear is not functionally active, it is still
possible, so long as the auditory nerve is normal, to hear vibra-
tions of a body (as a tuning-fork) held against the head;
though, as would be expected, discrimination as to pitch is
very imperfect.
Auditory impulses originate within the inner ear — that is
to say, in the vestibule and possibly the semicircular canals,
but especially in the cochlea. It is to he remembered that the
Fig. til.— Diagram intended to illustrate relative position of various parts of ear (after
Huxley). K. M, external auditory meatus; Tij. .1/. tympanic membrane; /'>/. tvm-
panum; Mall, mallens; Tnc, incus; Stp, stapes; F. o, fenestra ovalis; F.r, fenes-
tra rotunda; Si*, Eustachian tube; M. L, membranous labyrinth, only one of the
semicircular canals and its ampulla being represented; Sea. V,Sca. T. Sea. \f,
scahc of cochlea, represented as straight umcoiled).
566 COMPARATIVE PHYSIOLOGY.
whole of the end-organ concerned in hearing is bathed by endo-
lymph ; and that the vibrations of the latter are originated by
corresponding vibrations of the perilymph, which again is sent
Fig. 412.— Photographic diagram of labyrinth (after Flint and Rudinger). Upper fig-
ure: 1, utricle; 2, saccule; 3,5, membranous cochlea; 4, canalis reuniens; 6, semi-
circular canals. Lower figure: 1, utricle; 2, saccule; 3,4,6, ampulla?; 5,7.8,9,
semicircular canals; 10, auditory nerve (partly diagrammatic); 11, 12, 13, 14, 15, dis-
tribution of branches of nerve to vestibule and semicircular canals.
into oscillation by the movements of the stapes against the
membrane covering the fenestra ovalis ; so that the vibrations
thus set up without the membranous labyrinth are ti*ans-
formed into similar ones within the vestibule and the scala
vestibuli, and end, after passing over the scala tympani, against
the membrane of the fenestra rotunda. The cochlear canal
may be regarded as the seat of the most important part of the
HEARING.
567
organ of hearing, and answers to the macula lutea of the eye
in many respects.
The function of the organ of Corti is unknown.
The structure of the ampullae of the semicircular canals,
and other parts of the lahyrinth besides those specially con-
Fig. 413.— Distribution of cochlear nerve in spiral lamina of anteroinferior part of
cochlea of right ear (after Sappey). 1, trunk of cochlea nerve; 2, membranous
zone of spiral lamina; 3, terminal expansion of cochlear nerve exposed through-
out by removal of superior plate of lamina spiralis; 4, orifice of communication
between scala tympani and scala vestibuli.
sidered, with their peculiar hair-cells, suggests an auditory
function ; but what that may be is as yet quite undetermined.
It has been thought that the parts, other than the cochlea, are
concerned with the appreciation of noise, or perhaps the in-
tensity of sounds ; but this is a matter of pure speculation.
AUDITORY SENSATIONS, PERCEPTIONS, AND
JUDGMENTS.
We have thus far been concerned with the conduction of
the aerial vibrations that are the physical cause of hearing ;
but before we can claim to have " heard " a word in the highest
sense, certain processes, some of them physiological and some
psychical, take place, as in the case of vision ; hence we may
speak of the affection of the end-organ or of auditory impulses,
and of the processes by which these become, by the mediation
of the brain, auditory sensations, and when brought under the
cognizance of the mind as auditory perceptions and judg-
ments.
568
COMPARATIVE PHYSIOLOGY.
Auditory Judgments. — Such are much more frequently erro-
neous than are our visual judgments, whether the direction or
the distance of the sound he considered. As in the case of the
eye, the muscular sense, from accommodation of the vibratory
mechanism, may assist our judgments, being aided by our
stored past experiences (memory) according to the law of asso-
ciation. Sounds are, however, always referred to the world
without us. The animals with movable ears greatly excel man
in estimating the direction, if not the distance, of sounds. There
are few physiological experiments more amusing than those
performed on a person blindfolded, when attempting to deter-
mine either the distance or the direction of a sounding tuning-
fork, so gross are the errors made.
One who makes such observations on others may notice that
most persons move the ears slightly when attempting to make
the necessary discriminations, which of itself tends to show how
valuable mobility of these organs must be to those animals that
have it highly developed.
SPECIAL CONSIDERATIONS.
Comparative. — Among invertebrates steps of progressive de-
velopment can be traced. Thus, in certain of the jelly-fishes
we find an auditory vesicle (Fig.
414) inclosing fluid provided with
one or more otoliths or calcareous
nodules and auditory cells with at-
tached cilia, the whole making up
an end-organ connected with the
auditory nerve. A not very dis-
similar arrangement of parts exists
in certain mollusks (Fig. 415). The
vesicle may lie on a ganglion of
the central nervous system. On
the other hand, the vesicle may be
open to the exterior, as in decapod
crustaceans ; and the otoliths be re-
placed by grains of sand from with-
out. It is difficult to decide what
the function of otoliths may be in
Fie. 414.— Auditory vesicle of Gery-
onia (Garmarin a) seen from the
surface (after O. and R. Hert-
wigV N and N', the auditory
nerves: Ot, otolith; llz, audito-
ry cells; ////, auditory cilia (type mammals ; but there seems to be
of the auditory organ of the
TrachymedUBdi) .
little reason to doubt that they com-
HEARING.
569
municate vibrations in the invertebrates. When the cephal-
opod mollusks, with their highly developed nervous system,
are reached, we find a membranous and cartilaginous labyrinth.
Among vertebrates the different parts of the mammalian
ear are found in all stages of development. The outer ear may
be wholly wanting, as in the frog, or it may exist as a meatus
only, as in birds. The tympanic cavity is wanting in snakes.
Most fishes have a utricle and three semicircular canals, but some
Fig. 415. — Auditory vesicle of a heteropod mollusk (Pterotrachea) (after Claus). V,
auditory nerve; Of, otolith in fluid of vesicle; Wz, ciliated cells on inner wall of
vesicle; Hz, auditory cells; Cz, central cells.
have only one ; and the lowest of this group have an ear not
greatly removed from the invertebrate type, as may be seen in
the lamprey, which has a saccule with auditory hairs and oto-
liths, in communication with two semicircular canals. Most
of the amphibia are without a membrana tympani. The frog
has (1) a membrana tympani communicating with the inner ear
by (2) a bony and cartilaginous lever (columella auris), and (3)
an inner ear consisting of three semicircular canals, a saccule
and utricle containing many otoliths, and a small dilatation of
the vestibule, which may indicate an undeveloped cochlea.
The membranous labyrinth is contained in a periotic capsule,
partly bony and partly cartilaginous, which is supplied with
570
COMPARATIVE PHYSIOLOGY.
Fig. 416.— Otoliths from various animals (after Eiidinger). 1, from goat; 2, herring;
3, devil-fish; 4, mackerel; 5, flying-fish; 6, pike; 7, carp; 8, ray; 9, shark; 10,
grouse.
G.C.
Fio. 417.— Transverse section through head of foetal sheep, in region of hind-brain, to
illustrate development of ear (after BOtteher). //. B, hind-brain; N, auditory
nerve; V. JB, vertical semicircular canal; (!(', canal of cochlea; Ii. V, recessus
vestibuli; G, C, auditory ganglion; (/', terminal portion of auditory nerve.
HEARING. 571
perilymph. There is a fenestra ovalis, but not a fenestra ro-
tunda, though the latter is present in reptiles. In crocodiles
and birds the cochlea is tubular, straight, and divided into a
scala tympani and a scala vestibuli. The columella of lower
forms still persists. In birds and mammals the bone back of
the ear is hollowed out to some extent and communicates with
the tympanum. Except among the very lowest mammals
(Echidna), the ear is such as has been described in detail
already.
Evolution. — The above brief description of the auditory organ
in different groups of the animal kingdom will suffice to show
that there has been a progressive development or increasing
differentiation of structure, while the facts of physiology point
to a corresponding progress in function — in other words, there
has been an evolution. No doubt natural selection has played
a great part. It has been suggested that this is illustrated by
cats, that can hear the high tones produced by mice, which
would be inaudible to most mammals ; and, as the very exist-
ence of such animals must depend on their detecting their prey,
it is possible to understand how this principle has operated to
determine even what cats shall survive. The author has noticed
that terrier dogs also have a very acute sense of hearing, and
they also kill rats, etc. But, unless it be denied that the im-
provement from use and the reverse can be inherited, this factor
must also be taken into the account.
There seem to be great differences between hearing as it exists
in man and in lower forms. Birds, and at least some horses,
possibly some cats and dogs, like music, and give evidence of
the possession of a sense of rhythm, as evidenced by the conduct
of the steed of the soldier. On the other hand, some dogs seem to
greatly dislike music. Certain animals that appear to be devoid
of true hearing, as spiders, are nevertheless sensitive to aerial
vibrations ; whether by some special undiscovered organ or
through the general cutaneous or other kind of sensibility is
unknown. It also seems to be more than probable that some
groups of insects can hear sounds quite inaudible to us, though
by what organs is in great measure unknown.
The so-called musical ear differs from the non-musical in
the ability to discriminate differences in pitch rather thau in
quality; in fact, that one defective in the former power may
possess the latter in a high degree is a fact that has been some-
what lost sight of, both theoretically and practically. It does
572 COMPARATIVE PHYSIOLOGY.
not at all follow that one with little capacity for tune may not
have the qualifications of ear requisite to make a first-rate elo-
cutionist. Following custom, we have spoken as though certain
defects and their opposites depended on the ear, but in reality
we can not, in the case of man at all events, affirm that such is
the case ; indeed, it seems, on the whole more likely that they
are cerebraF or mental. Auditory discriminations seem to be
equally if not more susceptible of improvement by culture than
visual ones, especially in the case of the young.
A " good ear " seems to depend in no small degree on mem-
ory of sounds, though the latter may again have its basis in
the auditory end-organs or in the cerebral cortex, as concerned
in hearing. The necessity for the close connection between the
co-ordinations of the laryngeal apparatus in singing and speak-
ing and the ear might be inferred from the fact that many ex-
cellent musicians are themselves unable to vocalize the music
they perfectly appreciate.
Synopsis of the Physiology of Hearing.— The ear can appre-
ciate differences in pitch, loudness, and quality of sounds,
though whether different parts of the inner ear are concerned in
these discriminations is unknown. Hearing is the result of a
series of processes, having their physical counterpart in aerial
vibrations, which begin in the end-organ in the labyrinth and
terminate in the cerebral cortex. We recognize conducting
apparatus which is membranous, bony, and fluid. The auditory
nerve conveys the auditory impulses to the brain, though ex-
actly what terminal cells are concerned and how in originating
them must be regarded as undetermined. The essential part of
the organ of hearing is bathed by endolymph, and the princi-
pal part (in mammals) is within the cochlear canal. Man's
power to locate sounds is very imperfect. The auditory brain
center (or centers) has not been definitely located. Compara-
tive anatomy and physiology point clearly to a progressive
development of the sense of hearing.
THE SENSES OF SMELL AND TASTE.
SMELL.
The nose internally may be divided into a respiratory and
an olfactory region. The latter, which corresponds, of course,
with the distribution of the olfactory nerve, embraces the upper
and part of the middle turbinated bone and the upper part of
the septum, all of which differ in microscopic structure from
the respiratory region. Among the ordinary cylindrical epi-
thelium of the olfactory region are found peculiar hair-cells
highly suggestive of those of the labyrinth of the ear, and
Fig. 418.— Parts concerned in smell (after Hirschfeld). 1, olfactory ganglion and
nerves; 2, branch of nasal nerve, distributed over the turbinated bones."
which are to be regarded as the en d- organs of smell. If arom atic
bodies be held before the nose, and respiration suspended, they
will not be recognized as such, and it is well known that sniff-
574
COMPARATIVE PHYSIOLOGY.
ing greatly assists the sense of smell. Again, if fluids, such, as
eau de Cologne, he held in the nose, their aroma is not detected ;
and immediately after water has
heen kept in the nostrils for a few
seconds, it may he noticed that
smell is greatly hlunted. Such is
the case also when the mucous
membrane is much swollen from
a cold. There can be no doubt
that the presence of fluid in the
above cases is injurious to the del-
icate hair-cells, and that smell is
dependent upon the excitation of
these cells by extremely minute
particles emanating from aromatic
bodies.
When ammonia is held before
the nose, a powerful sensation is
Fig. 419.— End-organs concerned m ' *
smell (after koiiiker). i, from experienced ; but this is not smell
frog — a, epithelial cell of the _, . . -.
olfactory area; b, olfactory cell, proper, but an affection ot orcti-
i"?SSS5S*Sf5?SSSY£SSS *ary sensation, owing to stimula-
of varicose fibers. 3, olfactory ^ f ^ terminals of the fifth
cell of sheep.
nerve. It is possible that the audi-
tory nerve may also participate, though certainly not so as to
produce a pure sensation of smell.
Like the other sense-organs, that of smell is readily fa-
tigued ; and perhaps the satisfaction from smelling a bouquet
of mixed flowers is comparable to viewing the same, one scent
after another being perceived, and no one remaining predomi-
nant.
Our judgment of the position of bodies possessing smell is
less perfect even than for those emitting sounds ; but we always
project our sensations into the outer world, never referring the
object to the nose itself. Subjective sensations of smell are
rare in the normal subject, though common enough among
the diseased, as is complete or partial loss of smell. It has
been found that injury to the fifth nerve interferes with smell,
which is probably due to trophic changes in the olfactory
region.
Comparative.— The investigation of the senses in the lower
forms of life is extremely difficult, and in the lowest presents
almost insurmountable obstacles to the physiologist because
SENSES OF SMELL AXD TASTE. 575
their psychic life is so far removed from our own in terms of
which we must interpret, if at all.
The earliest form of olfactory organ appears to be a depres-
sion lined with special cells in connection with a nerve, which,
indeed, suggests the embryonic beginnings of the olfactory
organ in vertebrates, as an involution (pit) on the epithelium
of the head region. It would appear that we must believe that
in some of the lower forms of invertebrates the senses of smell
and taste are blended, or possibly that a perception results
which is totally different from anything known to us. The
close relation of smell and taste, even in man, will be referred
to presently, There are, perhaps, greater individual differences
in sensitiveness of the nasal organ among mankind than of any
other of the sense-organs. Women usually have a much keener
perception of odors than men. The sense of smell in the dog
is well known to be of extraordinary acuteness ; but there are
not only great differences among the various breeds of dogs,
but among individuals of the same breeds; and this sense is
being constantly improved by a process of " artificial selection "
on the part of man, owing to the institution of field trials for
setters and pointers, the best dogs for hunting (largely deter-
mined by the sense of smell) being used to breed from, to the
exclusion of the inferior in great part. Our own power to
think in terms of smell is very feeble, and in this respect the
dog and kindred animals probably have a world of their own
to no small extent. Their memory of smells is also immeasur-
ably better than our own. A dog has been known to detect an
old hat, the property of his master, that had been given away
two years before, as evidenced by his recovering it from a re-
mote place.
The importance of smell as a guide in the selection of food,
in detecting the presence of prey or of enemies, etc., is very
obvious. By culture some persons have learned to distinguish
individuals by smell alone, like the dog, though to a less degree.
TASTE.
The tongue is provided with peculiar modifications of epi-
thelial cells, etc., known as papillae and taste-buds which may
be regarded as the end-organs of the glosso-pharyngeaJ and
lingual nerves ; though that these all, especially the taste-buds,
are concerned with taste alone seems more than doubtful. In
576
COMPARATIVE PHYSIOLOGY,
certain animals with rough, tongues, the papillae, certain of
them at least, answer to the hairs of a brush for the cleansing
and general preservation of the coat of the animal in good con-
dition. We may, perhaps, speak of certain fundamental taste-
perceptions, such as siceet, bitter, acid, and saline. Certainly
the natural power of gustatory discrimination is considerable;
Fig. 420.— Papillae of tongue (after Sappey). 1, circumvallate papillae; 3, fungiform
papillae; I, filiform papillae; 0, glands at base of tongue; 7, tonsils.
and. as in the case of tea-tasters, capable of extraordinary culti-
vation. All parts of the tongue are not equally sensitive, nor
SENSES OF SMELL AND TASTE.
577
is taste-sensation confined entirely to the tongue. It can be
shown that the back edges and tip of the tongue, the soft palate,
the anterior pillars of the fauces, and a limited portion of the
back part of the hard palate, are concerned in tasting. Making
allowances for individual differences, it may be said that the
back of the tongue appreciates best bitter substances, the tip,
sweet ones, and the edges acids.
If any substance with a decided taste be placed upon the
tongue when wiped quite dry, it can not be tasted at all, show-
ing that solution is essential.
If a piece of apple, another of potato, and a third of onion,
be placed upon the tongue of a person blindfolded, and with
the nostrils closed, he will not be able to distinguish them,
showing that the senses of smell and of taste are related ; or,
perhaps, it may be said that much that we call tasting is in
large part smelling. When the electrodes from a battery are
placed on the tongue, a sensation of taste is aroused, described
differently by different persons ; also when the tongue is quick-
ly tapped, showing that, though taste is usually the result of
chemical stimulation, it may be excited by such as are electrical
or mechanical.
But it is not to be forgotten that we have usually no pure
gustatory sensations, but that these are necessarily blended
Fig. 421.
Fig. 483.
Fig. 421.— Medium-sized circumvallate papilla (after Sappcv).
Fig. 422.— Various kinds of papilhe cafter Sappey). 1. fungiform; 2. 3, 4. 5 6 filiform-
7, hemispherical papillae.
with those of common sensation, temperature, etc.. and that our
.-judgments must, in the nature of the case, be based upon highly
37
578
COMPARATIVE PHYSIOLOGY.
complex data, even leaving out of account other senses, such as
vision.
The glosso-pharyngeal is the principal nerve for the back of
the tongue, and for the tip the lingual ; or according to some
special fibers in this nerve, derived from the chord tympani.
It is worthy of note that temperature has much to do with
gustatory sensations, a very low or a very high temperature
Fig. 423. — Taste-buds from tongue of rabbit (after Engelmann).
being fatal to nice discrimination, and, as would be expected, a
temperature not far removed from " body-heat " (40° C) is the
most suitable.
A certain amount of pressure is favorable to tasting, as any
one may easily determine by simply allowing some solution of
quinine to rest on the tongue, and comparing the sensation with
that resulting when the same is rubbed into the organ ; hence
the importance of the movements of the tongue in appreciating
the sapid qualities of food.
Comparative. — Among the lowest forms of life it is extremely
difficult to determine to what extent taste and smell exist sepa-
rately or at all, as we can conceive of them. The differentia-
tion between ordinary tactile sensibility and these senses has
no doubt been very gradually effected. Observations on our
domestic animals show that their power of discrimination by
taste as well as by smell is very pronounced, though their likes
and dislikes are so different from our own in many instances.
At the same time we find that they often coincide, and it is not
unlikely that a dog's power of discriminating between a good
beefsteak and a poor one is quite equal if not superior to man's,
and certainly so if his sense of taste, as in the human subject, is
developed in proportion to his smelling power.
THE CEREBRO-SPINAL SYSTEM OF XEEVES.
I. SPINAL NERVES.
These (thirty-one pairs), which leave the spinal cord through
the intervertebral foramina, are mixed nerves — i. e., their mam
trunks consist of motor and sensory fibers. But before they
enter the spinal cord they separate into two groups, which are
Fig. 424. — Diagram of roots of spinal nerve illustrating: effects of section (after Dal-
ton). The dark regions indicate the degenerated parts.
known as the anterior or motor and the posterior or sensory
roots, which make connection with the anterior and posterior
gray horns respectively.
These facts have been established by a few simple but im-
portant physiological experiments, which will now be briefly
described : 1. Stimulation of the peripheral end of a spinal
nerve gives rise to muscular movements ; while stimulation of
its central end causes pain. 2. Upon section of the anterior
root, stimulation of its central end gives negative results ; but
of its peripheral end causes muscular movements. 3. After
section of the posterior root stimulation of the distal end is fol-
lowed by no sensory or motor effects ; of its central end, by
sensory effects (pain).
These experiments show clearly that the anterior roots are
motor, the posterior sensory, and the main trunk of the nerve
made up of mixed motor and sensory fibers.
580 COMPARATIVE PHYSIOLOGY.
Exception. — It has been found that sometimes stimulation of
the peripheral end of the anterior root has given rise to pain,
an effect which disappears if the posterior root be cut. From
this it is inferred that certain sensory fibers turn up into the
anterior root a certain distance. Such are termed " recurrent
sensory fibers."
Additional Experiments.— 1. It is found that if the anterior
root be cut, the fibers below the point of section degenerate,
while those above it do not. 2. On the other hand, when the
posterior root is divided above the ganglion, the fibers toward
the cord degenerate, while those on either side of the ganglion
do not. From these experiments it is inferred that the cells of
the posterior ganglion are essential to the nutrition of the sen-
sory fibers, and those of the anterior horn of the cord to the
motor fibers.
Pathological. — Pathology teaches the same lesson, for it is
observed that, when there is disease of the anterior gray cornua,
degeneration of motor fibers is almost sure to follow. These
cells, whether in the ganglion or the anterior horn, have been
termed "trophic." It is true, the functions of the ganglia on
the posterior roots, other than those just indicated, are un-
known ; on the other band, the cells of the anterior horn are
distinctly motor in function. To assume, then, that the cells of
the ganglion are exclusively trophic, with the evidence now
before us, would be premature.
The view we have presented of the relation of the nervous
system makes all cells trophic in a certain sense ; and we think
the view that certain cells or certain fibers are exclusively tro-
phic must, as yet, be regarded as an open question.
It is important, however, to recognize that certain connec-
tions between the parts of the nervous system, and indeed all
of the tissues, are essential for perfect "nutrition," if we are to
continue the use of that term at all.
II. THE CRANIAL NERVES.
These nerves have been divided into nerves of special sense,
motor, and mixed nerves.
The first class has already been considered, with the senses
to which they belong.
The physiology of the cranial nerves has been worked out
by means of sections and clinico-pathological investigations.
THE CEREBRO-SPINAL SYSTEM OP NERVES. 581
Speaking generally, a good knowledge of the anatomy of these
nerves is a great step toward the mastery of what is known of
Corpus (anticum
quadri- \
geminum(poshcum
Locus cocruleus
Eminentia teres
Crus cercbt'lli
ad pontem
Ala cinerea
Acccssorius nucleus
Brachinm conjunctivum anticum
Brachium conjunctivum.
poslicum
Corpus rjeiiiculatum
mediate
Pedunculus cerebri
ad corpora qua-}
drigemina I Crus
ad mediillam I cerebelli
oblongatam ]
VII
Obex
Clavd
Funiculus cuneatus
Funiculus gracilis
Fig. 425. — Intended .0 show especially the origin of both deep and superficial cranial
nerves (after Landois). Roman characters are used to indicate the nerves as they
emerge, and Arabic figures their nuclei or deep origin.
their functions, and such will he assumed in this chapter, so that
the student may expect to find the treatment of the siibject
somewhat condensed.
The Motor-Oculi or Third Nerve. — With a deep origin in the
gray matter of the floor and roof of the aqueduct of Sylvius,
branches of distribution pass to the following muscles : 1. All
of the muscles attached to the eyeball, with the exception of the
external rectus and the superior oblique. 2. The levator pal-
pebrae. 3. The circular muscle of the iris. 4. The ciliary
muscle. Both the latter branches reach the muscles by the
ciliary nerves, as they pass from the lenticular (ciliary, ophthal-
mic) ganglion. The relation of the third nerve, as seen in the
582 COMPARATIVE PHYSIOLOGY.
changes of the pupil with the movements of the eyeballs, has
already been noticed.
Pathological. — It follows that section or lesion of the third
nerve must give rise to the following symptoms : 1. Drooping
of the upper lid (ptosis). 2. Fixed position of the eye in the
outer angle of the orbit (luscitas). 3. Immobility, with the dila-
tation of the pupil (mydriasis). 4. Loss of accommodation.
The Trochlear or Fourth Nerve. — This nerve, arising in the
aqueduct of Sylvius, passes to the superior oblique muscle.
Pathological. — Lesion of this nerve leads to peculiar changes.
As there is double vision, some alteration must have occurred
in the usual position of the globe of the eye, though this is not
easily seen on looking at a subject thus affected. The double
image appears when the eyes are directed downward, and ap-
pears oblique and lower than that seen by the unaffected eye.
The Abductor or Sixth Nerve. — Arising on the floor of the
fourth ventricle, it passes to the external rectus of the eyeball,
thus with the third and fourth nerve completing the innerva-
tion of the external ocular muscles (extrinsic muscles).
Pathological. — Lesion of this nerve causes paralysis of the
above-mentioned muscle, and consequently internal squint
(strabismus).
The Facial, Portia Dura, or Seventh Nerve.— It arises in a
gray nucleus in the floor of the fourth ventricle, and has an
extensive distribution to the muscles of the face, and may be
regarded, in fact, as the nerve of the facial muscles, since it sup-
plies (1) the muscles of expression, as those of the forehead,
eyelids, nose, cheek, mouth, chin, outer ear, etc., and (2) certain
muscles of mastication, as the buccinator, posterior belly of the
digastric, the stylohyoid, and also (3) to the stapedius, with
branches to the soft palate and uvula.
Pathological. — It follows that paralysis of this nerve must
give rise to marked facial distortion, loss of expression, and
flattening of the features, as well as possibly some deficiency in
hearing, smelling, and swallowing. Mastication is difficult,
and the food not readily retained in the mouth. Speech is
affected from paralysis of the lips, etc.
Secretory fibers proceed (1) to the parotoid gland by the
superficial petrosal nerve, thence (2) to the otic ganglion, from
which the fibers pass by the auriculotemporal nerve to the
gland.
Gustatory Fibers. — According to some, the chorda tympani
THE CEREBROSPINAL SYSTEM OF NERVES. 583
really supplies the fibers to the lingual nerve that are concerned
with taste.
It will thus be seen that the facial nerve has a great variety
of important functions, and that paralysis may be more or less
serious, according to the number of fibers involved.
The Trigeminus, Trifacial, or Fifth Nerve. — This nerve has
very extensive functions. It is the sensory nerve of the face :
but, as will be seen, it is peculiar, being a combination of the
motor and sensory ; or, in other words, has paths for both
afferent and efferent impulses. The motor and less extensive
division arises from a nucleus in the floor of the fourth ventricle.
The sensory, much the larger, seems to have a very wide origin.
The nerve-fibers may be traced from the pons Varolii through
the medulla oblongata to the lower boundary of the olivary body
and to the posterior horn of the spinal cord. This origin sug-
gests a resemblance to a spinal nerve, the motor root corre-
sponding to the anterior, and the sensory to a posterior root,
the more so as there is a large ganglion connected with the
sensory part of the nerve within the brain-case.
Efferent Fibers.— 1. Motor. — To certain muscles (1) of mas-
tication— temporal, masseter, pterygoid, mylohyoid, and the
anterior part of the digastric. 2. Secretory. — To the lachrymal
gland of the ophthalmic division of this nerve. 3. Vaso-motor.
— Probably to the ocular vessels, those of the mucous mem-
brane of the cheek and gums, etc, 4. Trophic. — From the re-
sults ensuing on section of this nerve, it has been maintained
that special trophic fibers pass in it. We have discussed this
subject in an earlier chapter.
Afferent Fibers. — 1. Sensory. — To the entire face. To par-
ticularize regions : 1. The whole of the skin of the face and
that of the anterior surface of the external ear. 2. The external
auditory meatus. 3. The mucous lining of the cheeks, the floor
of the mouth, and the anterior region of the tongue. 4. The
teeth and periosteum of the jaws. 5. The lining membrane of
the entire nasal cavity. 6. The conjunctiva, globe of the eye,
and orbit. 7. The dura mater throughout.
Many of these afferent fibers are, of course, intimately con-
cerned with reflexes, as sneezing, winking, etc. Certain secre-
tory acts are often excited through this nerve, as lachrymation,
when the nasal mucous membrane is stimulated : indeed, the
paths for afferent impulses giving rise to reflexes, including
secretion, are very numerous.
584
COMPARATIVE PHYSIOLOGY.
Gustatory impulses from the anterior end and lateral edges
of the tongue are conveyed hy the lingual (gustatory) branch
of this nerve. Many are of opinion, however, that the fibers
of the chorda tympani, which afterward leave the lingual to
unite with the facial nerve, alone con-
vey such impressions. The subject
can not be regarded as quite settled.
Tactile sensibility in the tongue is very
pronounced, as we have all experi-
enced when a tooth, etc., has for some
reason presented an unusual surface
quality, and become a source of con-
stant offense to the tongue.
The ganglia of the fifth nerve, so
far as the functions of their cells are
concerned, are enigmatical at present.
They are doubtless in some sense tro-
phic at least. With each of these are
nerve connections (" roots " of the gan-
glia), which seem to contain different
kinds of fibers. These ganglia are
connected with the main nerve-centers
by both afferent and efferent nerves,
and also with the sympathetic nerves
themselves. Some regard the ganglia
as the representatives of the sympa-
thetic system within the cranium.
I. The Ciliary (Ophthalmic, Len-
ticular) Ganglion. — Its three roots
are : 1. From the branch of the third
nerve to the inferior oblique muscle
(motor root). 2. From the nasal
branch of the ophthalmic division of
the fifth. 3. From the carotid plexus
of the sympathetic. The efferent
branches pass to the iris, are derived
chiefly from the sympathetic, and
cause dilatation of the pupil. There
are also vaso-motor fibers to the choroid, iris, and retina. The
afferent fibers are sensory, passing from the conjunctiva, cor-
nea, etc.
II. The Nasal or Spheno-Palatine Ganglion.— The motor
Fk;. 426.— Unipolar cell from
Gaeserian ganglion (after
Schwalbe). N, N, N, nuclei
of sheath; 7', liber branch-
ing at a node of Ranvier.
THE CEREBRO-SPINAL SYSTEM OF NERVES. 585
root is dei-ived from the facial through the great superficial
petrosal nerve; its sympathetic root from the carotid plexus.
Both together constitute the vidian nerve. It would seem that
afferent impulses from the nasal chambers pass through this
ganglion. The efferent paths are : 1. Motor to the levator pa-
lati and azygos uvulae. 2. Vaso-motor, derived from the sym-
pathetic. 3. Secretory to the glands of the cheek, etc.
III. The Otic Ganglion.— Its roots are : 1. Motor, from the
third division. 2. Sensory, from the inferior division of the
fifth. 3. Sympathetic, from the plexus around the meningeal
artery. It makes communication with the chorda tympani and
seventh, and supplies the parotid gland with some fine fila-
ments. Motor fibers mixed with sensory ones pass to the tensor
tympani and tensor palati.
IV. The Submaxillary Ganglion. — Its roots are: 1. Branch-
es of the chorda tympani, from which pass (a) secretory fibers to
the submaxillary and sublingual glands, (b) vaso-motor (dilator)
fibers to the vessels of the same glands. 2. The sympathetic,
derived from the supei'ior cervical ganglion, passing to the sub-
maxillary gland. It is also thought to be the path of vaso-con-
strictor fibers to the gland. 3. The sensory, from the lingual
nerve, supplying the gland substance, its ducts, etc.
Pathological. — 1. The motor division of the nerve, when
the medium of efferent impulses, owing to central disorder, may
cause trismus (locked-jaw) from tonic tetanic action of the mus-
cles of mastication supplied by this nerve. 2. Paralysis of the
same muscles may ensue from degeneration of the motor nuclei
or pressure on the nerve in its course. 3. Neuralgia of any of
the sensory branches may occur from a great variety of causes,
and often maps out very exactly the course and distribution of
the branches of the nerve. 4. Vaso-motor distui'bances are not
infrequently associated with neuralgia. Blushing is an evi-
dence of the normal action of the vaso motor fibers of the fifth
nerve. 5. A variety of trophic (metabolic) disturbances may
arise from disorder of this nerve, its nuclei of origin or its gan-
glia, such as grayness and loss of hair (imperfect nutrition),
eruptions of the skin along the course of the nerves, etc. Atro-
phy of the face, on one or both sides, gradual and progressive,
may occur. Such affections as well as others, point in the most
forcible manner to the influence of the nervous system over the
metabolism of the body.
The Glosso-pharyngeal or Ninth Nerve. — This nerve, to-
586 COMPARATIVE PHYSIOLOGY.
gether with, the vagus and spinal accessory, constitutes the
eighth pair, or rather trio. Functionally, however, they are
quite distinct.
The glosso-pharyngeal arises in the floor of the fourth ven-
tricle ahove the nucleus for the vagus. It is a mixed nerve
with efferent and afferent fibers : Efferent fibers, furnishing
motor fibers to the middle constrictor of the pharynx, stylo-
pharyngeus, levator palati, and azygos uvulae. 2. Afferent
fibers, which are the paths of sensory impulses from the base
of the tongue, the soft palate, the tonsils, the Eustachian tube,
tympanum, and anterior portion of the epiglottis. Stimulation
of the regions just mentioued gives rise reflexly to the move-
ments of swallowing and to reflex secretion of saliva.
This nerve is also the special nerve of taste to the back of
the tongue.
The Pneumogastric, Vagus, or Tenth Nerve.— Most of the
functions of this nerve have already been considered in previous
chapters.
In some of the lower vertebrates (sharks) the nerve arises
by a series of distinct roots, some of which remain separate
throughout. This fact explains its peculiarities, anatomical
and functional, in the higher vertebrates. In these there have
been concentration and blending, so that what seems to be one
nerve is really made up of several distinct bundles of fibers,
many of which leave the main trunk later.
It may be regarded as the most complicated nerve-trunk in
the body, and the distribution of its fibers is of the most exten-
sive character. Following our classification of efferent and
afferent, we recognize :
1. Efferent fibers, which are motor to an extensive tract in
the respiratory and alimentary regions.
Thus the constrictors of the pharynx, certain muscles of the
palate, the oesophagus, the stomach, and the intestine, receive
an abundant supply from this source. By the laryngeal nerves,
probably derived originally from the spinal accessory, the mus-
cles of the larynx are innervated. The muscles of the trachea,
bronchi, etc., are also supplied by the pneumogastric. It is
probable that vaso-motor fibers derived from the sympathetic
run in branches of the vagus. The relations of this nerve to
the heart and lungs have already been explained.
2. Afferent Fibers. — It may be said that afferent impulses
from all the regions to which efferent fibers are supplied pass
THE CEREBRO-SPINAL SYSTEM OF NERVES. 587
inward by the vagus. One of the widest tracts in the body
for afferent impulses giving- rise to reflexes is connected with
the nerve-centers by the branches of this nerve, as evidenced by
the niany well-known phenomena of this character referable to
the pharynx, larynx, lungs, stomach, etc., as vomiting, sneez-
ing, coughing, etc. This nerve plays some important part in
secretion, no doubt, but what that is has not been as yet well
established.
Pathological. — Section of both vagi, as might be expected,
leads to death, which may take place from a combination of
pathological changes, the factors in which vary a good deal
with the class of animals the subject of experiment. Thus, the
heart in some animals (dog) beats with great rapidity and tends
to exhaust itself. In birds especially is fatty degeneration of
heart, stomach, intestines, etc., liable to follow.
Paralysis of the muscles of the larynx renders breathing
laborious. From loss of sensibility food accumulates in the
pharynx and finds its way into the larynx, favoring, if riot
actually exciting, inflammation of the air-passages.
But it is not to be forgotten that upon the views we advocate
as to the constant influence of the nervous system over all parts
of the bodily metabolism, it is plain that after section of the
trunk of a nerve with fibers of such wide distribution and va-
ried functions the most profound changes in so-called nutrition
must be expected, as well as the more obvious functional de-
rangements ; or, to put it otherwise, the results that follow are
in themselves evidence of the strongest kind for the doctrine of
a constant neuro-metabolic influence which we advocate. It
will not be forgotten that the depressor nerve, which exerts re-
flexly so important an influence over blood-pressure, is itself
derived from the vagus.
The Spinal Accessory or Eleventh Nerve!— This nerve arises
from the medulla oblongata somewhat far back, and from the
spinal cord in the region of the fifth to the seventh vertebra.
Leaving the lateral columns, its fibers run upward between the
denticulate ligament and the posterior roots of the spinal nerve
to enter the cranial cavity, which as they issue from the cra-
nium subdivide into two bundles, one of which unites with the
vagus, while the other pursues an independent course to reach
the sterno-mastoid and trapezius muscles, to which they furnish
the motor supply; so that it may be considered functionally
equivalent to the anterior root of a spinal nerve. The portion
588 COMPARATIVE PHYSIOLOGY.
joining the vagus seems to supply a large part of the motor
fibers of that nerve.
Pathological. — Tonic contraction of the flexors of the head
causes wry -neck, and when they are paralyzed the head is drawn
to the sound side.
The Hypoglossal or Twelfth Nerve.— It arises from the low-
est part of the calamus scriptorius and perhaps from the olivary
body. The manner of its emergence between the anterior pyra-
mid and the olivary body, on a line with the anterior spinal
roots, suggests that it corresponds to the latter ; the more so as
it, is motor in function, though also containing some vaso-motor
fibers, in all probability destined for the tongue. Such sensory
fibers as it may contain are derived from other sources (vagus,
trigeminus). It supplies motor fibers to the tongue and the
muscles, attached to the hyoid bone.
Pathological. — Unilateral section of the nerve gives rise to
a corresponding lingual paralysis, so that when the tongue is
protruded it points to the injured side ; when being drawn in,
the reverse. Speech, singing, deglutition, and taste may also
be abnormal, owing to the subject being unable to make the
usual co-ordinated movements of the tongue essential for these
acts.
RELATIONS OF THE CEREBRO-SPINAL AND SYMPA-
THETIC SYSTEMS.
No division of the nervous system has been so unsatisfac-
tory, because so out of relation with other parts, as the sympa-
thetic. It was also desirable to attempt to co-ordinate the cere-
bral and spinal nerves in a better fashion ; and various attempts
in that direction have been made. Very recently a plan, by
which the whole of the nerves issuing from the brain and cord
may be brought into a unity of conception, has been proposed;
and, though it would be premature to pronounce definitely as
yet upon the scheme, yet it does seem to be worth while to lay
it before the student, as at all events better than the isolation
implied in the three divisions of the nerves which has been
taught hitherto.
Instead of the classification of nerves into efferent and affer-
ent, connected with the anterior and the posterior horns of the
gray matter of the spinal cord, another division has been pro-
posed, viz., a division of nerve-fibers and their centers of origin
THE CEREBRO-SPINAL SYSTEM OF NERVES. 589
in the gray matter for the supply of the internal and the exter-
nal parts of the hody — i. e., into splanchnic and somatic nerves.
The centers of origin of the splanchnic nerves are referred to
groups of cells in the gray matter of the cord around the cen-
Fig. 427.
Fig. 428.
Fig. 427. — Ganglion cell from sympathetic ganglion of frog; greatly magnified, and
showing both straight and coiled fibers (after Quain).
Fig. 428. — Multipolar ganglion cells from sympathetic system of man, highly magni-
fied (after Max Scnuftze). a, cell freed from capsule; b, inclosed within a "nu-
cleated capsule. In both the processes have been broken away.
tral canal ; while the somatic nerves spring from the gray cor-
nua and supply the integument and the ordinary muscles of
locomotion, etc. The splanchnic nerves supply certain muscles
of respiration and deglutition, derived from the embryonic
lateral plates of the mesoblast; the somatic nerves, muscles
formed from the muscle-plates of the same region.
It is assumed that the segmentation of the vertebrate and
invertebrate animal is related; and that segmentation is pre-
590 COMPARATIVE PHYSIOLOGY.
served in the cranial region of the vertebrate, as shown by the
nerves themselves.
The afferent fibers of both splanchnic and somatic nerves
pass into the spinal ganglion, situated in the nerve-root, which
may be regarded as stationary.
It is different with the anterior roots. Some of the fibers
are not connected with ganglia at all ;' others with ganglia not
fixed in position, but occurring at variable distances from the
central nervous system (these being the so-called sympathetic
ganglia) : thus, the anterior root-fibers are divisible into two
groups, both of which are efferent, viz., ganglionated and non-
ganglionated. The ganglionated belong to the splanchnic sys-
tem, and have relatively small fibers; the non-ganglionated
include both somatic and splanchnic nerves, composing the
ordinary nerve-fibers of the voluntary striped muscles of res-
piration, deglutition, and locomotion.
It would appear that these now isolated ganglia have been
themselves derived from a primitive ganglion mass situated on
the spinal nerves; so that the distinction usually made of gan-
glionated and non-ganglionated roots is not fundamental.
A spinal nerve is, then, formed of — 1. A posterior root, the
ganglion of which is stationary in position, and connected with
splanchnic and somatic nerves, both of which are afferent. 2.
An anterior root, the ganglion of which is vagrant, and con-
nected with the efferent small-fibered splanchnic nerves.
Among the lower vertebrates both anterior and posterior
roots pass into the same stationary ganglion. Such is also the
case in the first two cervical nerves of the dog.
Does the above-mentioned plan of distribution, etc., hold for
the cranial nerves ?
Leaving out the nerves of special sense (olfactory, optic, and
auditory), the other cranial nerves maybe thus divided: 1. A
foremost group of nerves, wholly efferent in man, viz., the
third, fourth, motor division of the fifth, the sixth, and seventh.
2. A hindmost group of nerves of mixed character, viz., the
ninth, tenth, eleventh, and twelfth.
The nerves of the first group, since they have both large-
fibered, non-ganglionated motor nerves, and also small-fibered
splanchnic efferent nerves, with vagrant ganglia (ganglion
oculomotorii, ganglion geniculatum, etc.), resemble a spinal
nerve in respect to their anterior roots. They also resemble
spinal nerves as to their posterior roots, for at their exit from
THE CEREBRO-SPINAL SYSTEM OP NERVES. 591
the brain tliey pass a gang-lion corresponding- to the stationary-
posterior ganglion of the posterior root of a spinal nerve.
These being, however, neither in roots nor ganglion functional,
are to be regarded as the pbylogenetically (ancestrally) degen-
erated remnants of what were once functional ganglia and
nerve-fibers ; in other words, the afferent roots of these nerves
and their ganglia have degenerated.
The hindmost group of cranial nerves also answers to the
spinal nerves. They arise from nuclei of origin in the medulla
and in the cervical region of the spinal cord, directly continu-
ous with corresponding groups of nerve-cells in other parts of
the spinal cord ; but in these nerves there is a scattering of the
components of the corresponding spinal nerves. Cei'tain pecul-
iarities of these cranial nerves seem to become clearer if it be
assumed that, in the development of the vertebrate, degenera-
tion of some region once functional has occurred, in conse-
quence of which certain portions of nerves, etc., have disap-
peared or become functionless.
It is also to be remembered thafsa double segmentation ex-
ists in the body, viz., a somatic, represented by vertebras and
their related muscles, and a splanchnic represented by visceral
and branchial clefts, and that these two have not followed the
same lines of development ; so that in comparing spinal nerves
arranged in regard to somatic segments with cranial nerves,
the relations of the latter to the somatic muscles of the head
must be considered; in other words, like must be compared
with like.
THE VOICE,
It is convenient to speak, in the case of man, of the singing
voice and the speaking voice, though there is no fundamental
difference in their production. The voice of the lower animals
approximates the former leather than the latter.
It is to be remembered that sound is an affection of the
nervous centers through the ear, as the result of aerial vibra-
tions.
We are now to explain how such vibrations are caused by
the vocal mechanisms of animals and especially of man.
The toues of a piano or violin are demonstrably due to the
vibrations of the strings ; of a clarionet to the vibration of its
reed. But, however musical tones may be produced, we distin-
guish in them differences in pitch, quantity, and quality.
The pitch is dependent solely upon the number of vibrations
within a given time, as one second; the quantity or loudness
upon the amplitude of the vibrations, and the quality upon the
form of the vibrations. The first two scarcely require any fur-
ther notice ; but it is rather important for our purpose to under-
stand clearly the nature of quality or timbre, which is a more
complex matter.
If a note be sounded near an open piano, it may be observed
that not only the string capable of giving out the correspond-
ing note passes into feeble vibration, but that several others
also respond. These latter produce the overtones or partials
which enter into notes and determine the quality by which one
instrument or one voice differs from another. In other words,
every tone is in reality compound, being composed of a funda-
mental tone and overtones. These vary in number and in rela-
tive strength with each form of instrument and each voice;
and it is now customary to explain the differences in quality of
voices solely in this way ; and this is, no doubt, correct in the
main.
THE VOICE,
593
What are the mechanisms by which voice is produced in
man ? Observation proves that the following are essential : 1.
Fig. 429.
Fig. 430.
Fig. 429. — Longitudinal section of human larynx (after Sappey). 1, ventricle of lar-
ynx; 2, superior vocal cord; 3, inferior vocal cord; 4. arytenoid cartilage; 5, sec-
tion of arytenoid muscle; 6. 6. inferior portion of cavity of larynx: ',. section Of
posterior part of cricoid cartilage: 8. section of anterior part of same; 9, superior
border of cricoid cartilage; 10, section of thyroid cartilage; 11.11. superior portion
of cavity of larynx: 12, 13, arytenoid gland; 14, 16, epiglottis; 15, 17, adipose tissue:
18. section of liyoid bone; 19, 19. SO, trachea.
Pig. 430. — Posterior aspect of muscles of human larynx (after Sappey). 1, posterior
crico-arytenoid muscle; 2, 3, 4, different fasciculi' of arytenoid muscle; 5, aryteno-
epiglottidean muscle.
A certain amount of tension of the vocal cords (bands). 2. A
certain degree of approximation of their edges. 3. An expira-
tory blast of air.
It will be noted that these are all conditions favorable to the
vibration of the vocal bauds. The greater the tension the
higher the pitch ; and the more occluded the glottic orifice the
more effective the expiratory blast of air.
The principle on which the vocal bands act may be illus-
38
594
COMPARATIVE PHYSIOLOGY.
tratecl in the simplest way by a well-known toy, consisting of
an elastic bag tied upon a hollow stem of wood, across which
rubber bands are stretched, and the vibration of which caused
by the air within the distended bag gives rise to the note,
It is especially important to recognize the nature, extent, and
Fig. 431.
Fig. 432.
Fig. 431.— Lateral view of laryngeal muscles (after Sappey). 1, body of hyoid hone;
2, vertical section of thyroid cartilage; 3, horizontal section of thyroid cartilage,
turned downward to show deep attachment of crico-thyroid muscle; 4, facet of
the articulation of small cornu of thyroid cartilage with cricoid cartilage; 5, facet
on cricoid cartilage; 6, superior attachment of crico-thyroid muscle; 7, posterior
crico-arytenoid muscle; 8, lateral crico-arytenoid muscle; 9, thyro-arytenoid mus-
cle; 10, arytenoid muscle proper; 11, aryteno-epiglottidean muscle; 12, middle
thyro-hyoid ligament; 13, lateral thyro-hyoid ligament.
Fig. 432. — Distribution of nerves in larynx of horse (Chauvean, after Toussaint). a,
base of tongue; b, epiglottis; c, arytenoid muscles; d, section of thyroid cartilage
to show pails it, covers; e, cricoid cartilage; /, trachea; g, (esophagus; h, thyro-
arytenoid muscle; i, lateral crico-arytenoid muscle; j, posterior crico-arytenoid
muscle; k, arytenoid muscle; 1, superior laryngeal nerve; 2, inferior laryngeal; 3,
branches of superior laryngeal passing to epiglottis and tongue; 4, branches of
superior laryngeal passing to oesophagus; 5, very fine multiple anastomoses be-
tween two laryngeals; fi, tracheal branches; 7, branch to posterior crico-arytenoid
muscle; a portion is distributed, through the muscles, to subjacent mucous mem-
brane; lo, branch passing to arytenoid muscle; 11, oesophageal branch to aryte-
noid muscle; 11, (esophageal branch of pharyngeal nerve; it sometimes comes
from external laryngeal.
THE VOICE.
595
effect on the vocal bands of the movements of the arytenoid
cartilages. These are most marked around a vertical axis, giv-
ing rise to an inward and outward movement of rotation, but
Fig. 433. — Diagrammatic section of larynx to illustrate action of Posterior crico-aryte
noid muscle (after Landois). In this and the two following figures the dotted
lines indicate the new position of the parts owing to the action of the muscles
concerned.
there are also movements of less extent in all directions. It is
in fact through the movements of these cartilages to which the
Fir. 434.— Diagrammatic section of larynx to illustrate action of Arytenoids* jirc-
privs musbb (after Landois).
596
COMPARATIVE PHYSIOLOGY.
Fig. 435. — Illustrates action of thyro-arytenoideus interims.
vocal bands are attached posteriorly, that most of the important
changes in the tension, approximation, etc., of the latter are
produced. The lungs are to he regarded as the bellows furnish-
ing the necessary wind-power to set the vocal bands vibrating,
while the larynx has respiratory as well as vocal functions, as
has been already learned. Assuming that the student has a
good knowledge of the general anatomy of the larynx, we call
attention briefly to the following :
Widening of the glottis is effected by the crico-arytenoideus
posticus pulling outward the processus vocalis or attachment
posteriorly of the vocal band, and a similar effect is produced
by the arytenoideus posticus acting alone.
Narrowing of the glottis is accomplished by the crico-aryt-
enoideus lateralis, and the following when acting either singly
(except the arytenoideus posticus), or in concert, as the sphinc-
ter of the larynx, viz., the thyro-arytenoideus externus, thyro-
arytenoideus internus, thyro-aryepiglotticus arytenoideus pos-
ticus.
Tension of the vocal bands is brought about by the sphincter
group, and especially by the external and internal thyro-aryte-
noid muscles.
Nerve Supply. — The superior laryngeal contains the motor
fibers for the crico-thyroid (possibly also the arytenoideus pos-
ticus) and also supplies the mucous membrane. The inferior
laryngeal supplies all the other muscles. "While both of these
nerves are derived from the vagus, their fibers really belong to
the spinal accessory. It is worthy of note that the entire group
THE VOICE.
597
of muscles making up the sphincter of the larynx is contracted
when the inferior laryngeal is stimulated.
Superior Face. Inferior Face.
Fig. 436.— Cartilaginous pieces of the larynx of horse, maintained in their natural
position by the articular ligaments (Chauveau). a, cricoid cartilage; b. b, aryte-
noid cartilages; c, body of the thyroid; c', c', lateral plates of the thyroid; d. epi-
glottis; e, body of the hyoid; /, trachea. 1, crico-arytenoid articulation; 2, capsule
of the crico-thyroid articulation; 3, crico-thyroid membrane; 4, thyro-hyoid mem-
brane; 5, crico-trachealis ligament.
Above the true vocal bands composed of elastic fibers lie the
so-called false vocal bands (cords) to be regarded as folds of the
mucous membrane which take no essential part in voice-produc-
tion. Between these two pairs of bands are the ventricles of
Morgagni, which, as well as the adjacent parts, secrete mucus
and allow of the movements of both sets of bands and in so far
only assist in phonation.
The whole of the supra-laryngeal cavities, the trachea and
bronchial tubes, may be regarded as resonance-chambers, the
former of which are of the most importance, so far as the
quality of the voice is concerned. There seems to be little
doubt that they have much to do with determining the differ-
ences by which one individual's voice at the same pitch differs
from another ; nor is the view that they may have a slight in-
fluence on the pitch of the voice, or even its intensity, to be
ignored.
'598
COMPARATIVE PHYSIOLOGY.
The epiglottis, in so far as it has any effect, in all probability
modifies the voice in the direction of quality.
Pathological.— Paralysis of
the laryngeal muscles, owing to
pressure on nerves and conse-
quent narrowing of the glottic
opening, explains " roaring " in
the horse, in certain instances
at all events.
Comparative.— Much more
is known of the sounds emanat-
ing from the lower animals
than of the mechanisms by
which they are produced. This
applies, of course, especially to
such sounds as are not pro-
duced by external parts of the
body, it being very difficult to
investigate these experimental-
ly or to observe the animal
closely enough when produc-
ing the various vocal effects
naturally.
All our domestic mammals
have a larynx, not as widely different from that of man as
might be supposed from the feeble range of their vocal powers.
There are structural differences in the larynx of the domestic
animals, some of which are more readily appreciated by the eye
than described.
The false (superior) vocal bands are rudimentary or want-
ing in many mammals, including the horse, ass, etc.
In ruminants the larynx is proportionately ill-developed ;
the glottis is short, the vocal bands rudimentary, and the ven-
tricles wanting.
The larnyx of the pig is peculiar in that the ventricles are
deep, though their opening is only a narrow slit; there is, how-
ever, a large membranous sac below the epiglottis, which,
acting as a resonator, explains the great intensity of the voice
of this animal.
The actual behavior of the vocal bands has been studied
experimentally, in the dog when growling, barking, etc. And,
so far as it goes, this animal's mechanism of voice-production
Fig. 437.— Posterolateral view of the lar-
ynx of the horse (Chauveau). 1, epi-
glottis ; 2, arytenoid cartilages ; 3,
thyroid cartilage ; 4, arytenoideus
muscle; 5, crico-arytenoideus latera-
lis; 6, thyro-arytenoideus; 7, crico-
arytenoideus posticus: 8, crico-thy-
roidens; 9, ligament between the cri-
coid cartilage and first ring of trachea;
10. 11, infero-posterior extremities of
crico-thyroid cartilages.
THE VOICE.
599
is not essentially different from that of man. Growling is the
result of a functional activity of the vocal mechanism, not un-
like that of man when singing a bass note; barking, of that
analogous to coughing or laughing, when the vocal bands are
rapidly approximated and separated.
The grunting of hogs and the lowing and bawling of horned
cattle are probably very similar in production, so far as the
larynx is concerned, to the above. The cat has plainly very
great command over the larynx, and can produce a wide range
of tones. The peculiarities of the bray of the ass are owing to
voice production both during inspiration and expiration.
The quality of the voice of
most animals appears harsh to
our ears, owing probably "to a
great preponderance of over-
tones, in consequence of an im-
perfect and unequal tension of
the vocal bands; but the influ-
ence of the supra-laryngeal cavi-
ties, often very large, must also be
taken into account.
In certain of the primates, and
especially in the howling mon-
keys, large cheek-pouches can be Fig. 438-Lower larynx (Syrinx) of
inflated with air from the larynx,
and so add to the intensity of the
note produced by the vocal bands
that then* voice may be heard for
miles. Song-birds produce their
notes, as may be seen, by exter-
nal movements low down at the bifurcation of the trachea
(syrinx). The notes are owing to the vibration of two folds of
the mucous membrane, which project into each bronchus, and
are regulated in their movements by muscles, the bronchial
rings in this region being correspondingly modified.
A large number of species of fishes produce sounds and in
a variety of ways, in which the air-bladder, stomach, intestines.
etc., take part. Most reptiles are voiceless, in the proper sense,
though there are few that can not produce a sort of hissing
sound, caused by the forcible emission of air through the upper
respiratory passages.
Frogs, as is well known, produce sounds of great variety in
crow (after Gegenbaurl." A, seen
from side: B, seen from in front.
a—f, muscles concerned in move-
ments of lower larynx; rj. mem-
brana tympaniformis interna,
stretching from median surface of
either bronchus to a bony ridge
(.pessulus) which projects at the
angle of bifurcation of trachea.
(500 COMPARATIVE PHYSIOLOGY.
pitch, quality, and intensity, some species croaking so as to be
heard at the distance of at least a mile. It is a matter of easy
observation that when frogs croak the capacity of the mouth
cavity is greatly increased, owing to the distention of resonat-
ing sacs situated at each angle of the jaws. When tree-frogs
croak, their throats are greatly distended, apparently in suc-
cessive waves.
SPECIAL CONSIDERATIONS AND SUMMARY.
Evolution. — The very lowest forms, and in fact most inverte-
brate groups, seem to be voiceless. Darwin has shown that
voice is, in a large number of groups, confined either entirely
to the male, or that it is so much more developed in him as to
become what he terms a " sexual character." There is abundant
evidence that males are chosen as mates by the females, among
birds especially, not alone for superiority in beauty of plumage,
but also for their song. Thus, by a process of natural selection
(sexual selection), the voice would tend to improve with the
lapse of time, if we admit heredity, which is an undeniable fact,
even among men — whole families for generations, as the Bachs,
having been musicians.
One can also understand why on these principles voice
should be especially developed in certain groups (birds), while
among others (mammals) form and strength should determine
sexual selection, the strongest winning in the contests for the
possession of the females, and so propagating their species under
the more favorable circumstance of choice of the most desira-
ble females.
Pathology teaches that, when certain parts of the brain
(speech-centers) of man are injured by accident or disease, the
power of speech may be lost. From this it is evident that the
vocal appai-atus may be perfect and yet speech be wanting; so
that it becomes comprehensible that the vocal powers of, e. g.,
a dog, are so limited, notwithstanding his comparatively highly
developed larynx. He lacks the energizing and directive ma-
chinery situated in the brain.
Some believe that there was a period when man did not pos-
sess the power of speech at all ; and many are convinced that
the human race have undergone a gradual development in this
as in other respects. Certain it is that races differ still very
widely in capacity to express ideas by spoken words.
THE VOICE 601
We may regard the development of a race of speaking ani-
mals as dependent upon a corresponding advance in brain-
structure, whether that was acquired by a sudden and pro-
nounced variation, or by gradual additions of increase in cer-
tain regions of the brain, or whether to the first there was then
added the second.
Apart from speech proper, there is a language of the face
and body generally, in which there is much that we share with
lower forms, especially lower mammals. Darwin, noticing this
resemblance, regarded it as evidence strengthening the belief
that man is derived from lower forms. Why should the forms
of facial expression associated so generally with certain emotions
among different races of men be so similar to each other and
to those which the lower animals employ, if there is not some
community of origin ? This is Darwin's query, and he con-
sidered, as has been stated, that the answer to be given was in
harmony with his views of man's origin, as based on an alto-
gether different sort of testimony.
The high functional development of the hand and arm in
man, and the use of these parts in writing, are suggestive.
Summary. — The musical tones of the voice are caused by the
vibrations of the vocal bands, owing to the action on them of
an expiratory blast of air from the lungs. In order that the
bands may act effectively, they must be rendered tense and ap-
proximated, which is accomplished by the action of the laryn-
geal muscles, especially those attached to the arytenoid carti-
lages. We may speak of the respiratory glottis and the vocal-
izing glottis, according as we consider the position and move-
ments of the vocal bands in respiration or in phonation.
The pitch of the voice is determined by the length and the
tension of the vocal bands, and frequently both shortening and
increased tension are combined; perhaps we may say that al-
tered (not necessarily increased) tension and length are always
combined.
The quality of the voice depends chiefly upon the supi-a-
laryngeal cavities.
It is important to remember that in all phonation, in the
case of man at least, many parts combine to produce the result:
so that voice-production is complex and variable in mechanism,
beyond what would be inferred from the apparent simplicity of
the mechanism involved ; while the central nervous processes
are, when comparison is made with phonation in lower ani-
602 COMPARATIVE PHYSIOLOGY.
mals, seen to be the most involved and important of the whole
— a fact which the results of disease of the brain are well calcu-
lated to impress, inasmuch as interruptions anywhere among a
class of cerebral connections, now known to be very extensive,
suffice to abolish voice, and especially speech -production.
Among mammals below man the vocal bands and laryngeal
and thoracic mechanism are very similar, but less perfectly
and complexly co-ordinated ; so that their vocalization is more
limited in range, and their tones characterized by a quality
which to the human ear is less agreeable. Man's superiority as
a speaking animal is to be traced chiefly to the special develop-
ment of his cerebrum, both generally and in certain definite
regions.
CERTAIN TISSUES.
Prior to considering the subject of the next chapter, it may
be well to give a short account of certain tissues specially con-
cerned.
Connective Tissue.— This is the most widely distributed
tissue in the body, since it binds together all other forms of
tissue, and, in some of its many varieties, enters into the forma-
tion of every organ. As connective tissue proper, its function
is subordinate ; but when it becomes the aponeuroses of mus-
Fig. 439.— Fibers of tendon of man (Rollettt.
cles, and especially tendons, by which, from its inextensibility,
the muscles are rendered effective in moving the levers (bones)
to which they are attached, its importance is more pronounced.
In structure, this fibrous tissue consists of bundles of fine fibrils,
among which, especially in the younger stage, connective-tissue
cells may be found, and from which the Abel's themselves are
formed.
604
COMPARATIVE PHYSIOLOGY.
It is owing- to differences in the shape and size of these cells
chiefly that the structural variations of connective tissue in dif-
ferent regions of the body are due.
1 1
I
Fig. 440.— Loose network of connective tissue from man, in which are connective-tis-
sue corpuscles among the fibers (Rollet). a, a, capillary with blood-cells.
Elastic Tissue. — This form of tissue is also of very wide distri-
bution and of great importance in the economy of a complicated
living organism that must constantly adapt itself to the stress
and strains of existence. In its purest form it occurs, e. g., in
the ligamentum nuclese of the ox, as a somewhat yellow, tough,
elastic structure easily fibrillated when boiled, but with diffi-
culty torn asunder when fresh. Under the microscope it ap-
pears as fibers with a very distinct outline and of varying size.
In the arteries, as already referred to, it forms a sort of elastic
membrane of the utmost importance in the functions of these
organs.
Bone. — In a long bone, as the femur, in the dried state, we
recognize a compact shaft and two extremities of a more porous
nature, while the central portion of the former presents a more
or less circular cavity, the medullary canal. By treatment
with hydrochloric acid abundance of lime salts may be ob-
CERTAIN TISSUES.
605
^■^{^"y.n.; k
Fig. 433.
Fig. 441.
Fig. 442.
Fig. 441. — Fine elastic fibers from peritonaeum, 1 x 350 (Kolliker).
Fig. 442. — Larger elastic fibers (Robin).
Fig. 443. — Elastic network (fenestrated membrane) from middle coat of carotid of
horse, 1 x 350 (Kolliker).
ym^miMm ill*
8
n ip**;*tsiv,
': ty\W
Fig. 444.— Longitudinal section of humerus, showing Haversian canals and lacunre,
1 x 200 (Sappey).
606
COMPARATIVE PHYSIOLOGY.
Fig. 445. — Transverse section of humerus. 1 x 200 (Sappey). 1, section of vascular
(Haversian) cells; 2, longitudinal canal at point of junction with transverse canal.
Lacunae and canaliculi arranged in concentric rings.
tained. A microscopic transverse section shows the substance
of the shaft to be penetrated by longitudinal channels (Haver-
r r>
Pig. 446. — Bone-corpuscles and their processes which fill the lacunae and canaliculi
(Rollett).
CERTAIN TISSUES.
601
sian canals), while the intermediate space is occupied by cavi-
ties (lacunae) connected with one another by very fine canals
thl
Fig. 447. — Vertical section of articular cartilage resting on bone, and showing cells
and capsules arranged in layers as indicated by numerals (Sappey).
(Fig. 444). A vertical cross - section exhibits the lamella? of
which it is made up and the vascular channels cut across
(Fig. 445).
All this is, however, only the framework of osseous tissue.
If a bone from an animal freshly killed, without bleeding, be
examined, a very different state of things will be found. The
bone is heavier ; its surface is covered with a closely adherent,
tough, fibrous structure, the periosteum : and its medullary
cavity filled with marrow. If the bone be broken across, its
section looks red, and blood flows from the surface. Investiga-
tion proves that the covering periosteum is a bed in which
blood-vessels and nerves ramify, and from which they enter
608
COMPARATIVE PHYSIOLOGY-
the openings to be seen on the surface of the dead bone. The
Haversian canals are vascular channels, and the lacunae filled
with bone corpuscles (Fig. 446). The marrow in the extremi-
ties of the bone is of a red color in consequence of its great vas-
cularity; and in the young animal a similar marrow fills the
Pia. 448.— Section of cartilage of ear of man (Rollett). a, fibre-cartilage; b, connect-
ive tissue. The cartilage had been boiled and dried prior to cutting.
medullary canal, but later it is less vascular, and abounds in
fat. Blood-vessels pass from it into the compact tissue of the
CERTAIN TISSUES. 609
bone. The main artery, whence the others are derived, for the
shaft of the hone, enters by the nutrient foramen on the sur-
face, and toward the center.
The bone-corpuscles (Fig. 446), answering to the connective-
tissue cells, are nutritive and formative after a considerable
portion of the tissue has become the seat of the deposit of lime-
salts. Bone is a living tissue, though in a less degree than
most others ; but it is only by bearing these relations in mind
that its function in the support of the soft parts of an animal,
and especially as constituting the essential levers of its locomo-
tive mechanism, can be understood.
Cartilage. — In the earliest stages of an animal's existence
the bones are represented by cartilage, and at all periods of its
existence this structure forms those elastic pads that cover its
articular surfaces, and shield the bones and the entire animal
from, undue concussion. The kind of cartilage that covers the
extremities of the long bones, known as articular, is character-
ized by abundance of cells lying in a homogeneous bed or ma-
trix (Fig. 447).
Fibro-cartilage (Fig. 448) abounds in fibrous tissue, some
elastic fibers, characteristic cells, etc., and is found between the
bodies of the vertebrae and in similar situations, as well as in the
epiglottis, the ear, etc.
39
LOCOMOTION.
The entire locomotor system of tissues is derived from the
embryonic mesoblast. These include the muscles, hones, carti-
lage, and connective and fibrous tissues ; and the tissues that
make up the vascular system or the motor apparatus for the
circulation of the blood. Locomotion in the mammal is effected
by the movement of certain bony levers, while the equilibrium
of the body is maintained.
The whole series of levers is
bound together by muscles,
tendons, ligaments, etc., and
play over one another at cer-
tain points where they are in-
vested with cartilage, and
kept moist by a secretion from
the cells covering the syno-
vial membranes that form the
inner linings of joints.
Cartilage, a very low form
of tissue destitute of blood-
vessels, and hence badly re-
paired when lost by injury
or disease, forms a series of
smooth surfaces admirably
adapted for joints, and espe-
cially fitted to act as a series
of elastic buffers, and thus
prevent shocks. Bone, though
brittle in the dried state, possesses, when alive, a favorable de-
gree of elasticity, while sufficiently rigid. Provision is made
by its vascular periosteum and central marrow (in the case of
the long bones), as Avell as by the blood-supply derived from
the nutrient artery and its ramifications throughout the osseous
LOCOMOTION.
611
tissue, for abundant nourishment, growth, and repair after in-
jury-
We find in the body of mammals, including man, examples
of all three kinds of levers. It sometimes happens that there
is an apparent sacrifice of energy, the best leverage not being
exemplified ; but on closer examination it will be seen that the
weight must either be moved with nice precision or through
large distances, and these objects can not be accomplished al-
ways by the arrangements that would simply furnish the most
powerful lever. This is illustrated by the action of the biceps
on the forearm.
It is to be remembered that, while the flexors and extensors
of a limb act in a certain degree the opposite of one another,
Fig. 452. — Skeleton of deer. The bone.* in the extremities of this the fleetest of quad-
rupeds are inclined very obliquely toward each other and toward the scapular and
iliac bones. This arrangement increases the leverage of the muscular system and
confers great rapidity on the moving parts. It augments elasticity, diminishes
shock, and indirectly begets continuity of movement, n. angle formed by femur
with ilinm; 6, angle formed by tibia and fibula with femur; c, angle formed by
phalanges with cannon-bone; e, angle formed by humerus with scapula: /. angle
formed by radius and ulna with humerus (Pettigrew).
there is also, in all cases perhaps, a united action ; the one
set, however, preponderating over the other, and usually sev-
eral muscles, whether of the same or different classes, act to-
gether.
Standing itself requires the exercise of a large number of
612
COMPARATIVE PHYSIOLOGY.
similar and antagonistic muscles so co-ordinated that the line
of gravity falls within the area of the feet. An unconscious
animal falls, which is itself an evidence of the truth of the
above remarks.
The folio-wing statements in regard to the direction of the
line of gravity in man may prove useful : 1. That for the head
falls in front of the occipital articulation, as exemplified by the
nodding of the head in a drowsy person occupying the sitting
attitude. 2. That for the head and trunk together passes behind
a line joining the centers of the two hip-joints, hence the uncor-
rected tendency of the erect body of man is to fall backward.
3. That for the head, trunk, and thighs falls behind the knee-
joints somewhat, which would also favor falling backward
(bending of the knees). 4. The line of gravity of the whole
body passes in front of a line joining the two ankle-joints, so
Fig. 453.— Shows the simultaneous positions of both legs during a step, divided into
four groups (after Weber). First group (,4), 4 to 7, gives the different positions
which the legs simultaneously assume while both are on the ground; second group
(B), 8 to 11, shows the various positions of both legs at the time when the poste-
rior leg is elevated from the ground, but behind the supported one; third group
(C), 12 to 14, shows the positions which the legs assume when the swinging leg
overtakes the standing one; and the fourth group (/)), 1 to 3. the positions during
the time when the swinging leg is propelled in advance of the resting one. The
letters it. h, and c indicate the angles formed by (he bones of the right leg when
engaged in making a step; the letters m, n, and o, the positions assumed by the
right foot when the trunk is rolling over it; {/, shows the rotating forward of the
trunk upon the left foot (/) as an axis; h, shows the rotating forward of the left
leg and foot upon the trunk ((/) as an axis.
that the body would tend, but for the contraction of the mus-
cles of the calves of the legs, to fall forward.
Taking these different facts into consideration explains the
LOCOMOTION.
013
various directions in which an individual, when erect, may fall
according1 as one or the other line (centei'j of gravity is dis-
placed for a long enough time.
Walking (man) implies the alternate movement of each leg
forward, pendulum-like, so that for a moment the entire body
must be supported on one foot. "When the right foot is lifted
or swung forward, the left must support the weight of the
body. It becomes oblique, the heel being raised, the toe still
resting on the ground ; and it is upon this as a fulcrum that
the body -weight is moved forward, when a similar action is
taken up by the opposite leg.
It follows that to prevent a fall there must be a leaning of
the body to one side, so that the line of gravity may pass through
each stationary foot ; hence a person walking describes a series
of vertical curves with the head and of horizontal ones with the
body, the resulting total being complex.
Fig?. -154 and 455. — Showing the more or less perpendicular direction of the stroke of
the wing in the flight of the bird (gull); how the wing is gradually extended as it
is elevated (e,f, g)\ how it descends as a long lever until it assumes the position
indicated by A : how it is flexed toward thetermination of the down-stroke, as
shown at h'i.j. to convert it into a short lever («. ti) and prepare it for making the
lip-Stroke. The difference in the length of the w ing during flexion and extension
is indicated by the short and Jong levers a, b and c. d. The sudden conversion of
the wing froni a Ion;; into a short lever at the end of the down-stroke is of great
importance, as it robs the wing of its momentum and prepares it for reversing its
movements (Pettigrew).
The peculiarities of the gait of different persons are naturally
determined by their height, length of leg. and a variety of other
factors, which are often inherited with great exactness. We
instinctively adopt that gait which economizes energy, both
physical and mental.
Running differs from walking, in that both feet are for a
614
COMPARATIVE PHYSIOLOGY.
Figs. 450 and 457 show that when the wings are elevated («,/, g) the body falls («);
and that when the wings are depressed (h, i,j) the body is elevated (r). Fig. 456
shows that the wings are elevated as short levers (e) until toward the termination
of the up-stroke, when they are gradually expanded (/, g) to prepare them for
making the down-stroke. Fig. 457 shows that the wings 'descend as long levers
(A) until toward the termination of the down-stroke, when they are gradually
folded or flexed (i,j) to rob them of their momentum and prepare them for mak-
ing the up-stroke. (Compare with Figs. 454 and 455.) By this means the air be-
neath the wings is vigorously seized during the down-stroke, while that above it
is avoided during the up-stroke. The concavo-convex form of the wings and the
forward travel of the body contributes to this result. The wings, it will be ob-
served, act as a parachute both during the up and down strokes. Fig. 457 shows
also the compound rotation of the wing, how it rotates upon a, as a center, with
a radius m, b, n, and upon a, c, b as a center, with a radius k, I (Pettigrew).
period of the cycle off the ground at the same time, owing to a
very energetic action of the foot acting as a fulcrum.
Jumping implies the propulsion of the body by the impulse
given by both feet at the same moment.
Hopping is the same act accomplished by the use of one
leg.
Comparative. — Tbe movements of quadrupeds are naturally
very complicated, but have now been well worked out by the
use of instantaneous photography. Even the bird's flight is no
longer a wholly unsolved problem, but is fairly well under-
stood. The movements of centipedes and and other many-
legged invertebrates are highly complicated, while their rapid
movements are to be accounted for by the multiplicity of their
levers rather than the rapidity with which they are moved.
LOCOMOTION. 615
The length and flexibility of their bodies must also be taken
into account, rendering1 many legs necessary for support.
The subject of locomotion is of such great importance in the
practice of comparative medicine that we shall now enter upon
it in somewhat more detail, especially as regards the horse.
This, of all our domestic animals, has become specialized as a
locomotive mechanism. All the parts of his whole economy
have been co-ordinated to that end ; and, except the horse be
viewed in this light, the significance of much in his nature
sJfe^-
r —
Fig. 458.— Chillingham bull (Bos Scoticus). Shows powerful, heavy body, and the
small extremities adapted for land transit. Also the figure-of-8 movements made
by the feet and limbs in walking and running. ?/, t. curves made by right and
left anterior extremities; r, s, curves made by right and left posterior extremities.
The right fore and the left hind foot move together to form the waved line (s, w);
the left fore and the right hind foot move together to form the waved line (/•, t).
The curves formed by the anterior (t, u) and posterior (,/■, s) extremities form ellipses
(Pettigrew;.
will be missed. But, however well his other parts might be
suited to tbis purpose, unless the feet were adapted to rapid
movements and great and frequently repeated concussions, the
animal must soon break down. As it is, under the unnatural
conditions of our artificially constructed roads, faulty shoeing,
housing, and feeding, lamenesses of the feet constitute a large
proportion of the cases that fall under the care of the practitioner.
It may be well at the outset to give a little consideration to the
feet of the horse, in order to learn to what extent they are
adapted to natural conditions. The feet of all mammals illustrate
how the soft and yielding tissues are combined with the rigid,
to adapt to conditions of the surface over which they are re-
quired to move. In the carnivora, beneath the outer tough skin
covering the sole, there is the fatty cushion protective to the
bones and more delicate soft parts ; while the claws, nails, etc.
616
COMPARATIVE PHYSIOLOGY.
in which the toes end, are not only weapons of offense and de-
fense, but protective against injury from contact with hard sur-
faces, as well as directly helpful in locomotion. These princ
pies are admirably exemplified in the foot of solipeds.
The foot of the horse may be said to consist of terminal
bones incased in soft structures adapted to shield the animal
from the effects of excessive concussion and for nutrition, the
Fig. 459.
Fig. 460.
Fig. 430.— Longitudinal median section of foot. 1. anterior extensor of phalanges,
or extensor pedis; 2, lateral extensor, or extensor suffraginis; 3, capsule of meta-
carpophalangeal articulation; 4. large metacarpal bone; 5. superficial flexor of
phalanges, or perforatns; 0. deep flexor, or perforans; 7, sheath; 8, bursa; 9, sesa-
moid bone; 10, ergot and fatty cushion of fetlock; 11, crucial ligament; 12, short
sesamoid ligament; 13. first phalanx; 14, bursa; 15, second phalanx; 16, navicu-
lar bone; 17. plantar cushion; 18, third phalanx; 19, plantar surface of hoof ; 20,
sensitive or keratosrenous membrane of third phalanx.
Fig. 460.— Horizontal section of horse's foot. 1. front or toe of hoof; 2, thickness of
wall: 3. laminae; 4, insertion of extensor pedis; 5, os pedis; 6, navicular bone; 7,
u -JiiL's of os pedis; 8, lateral cartilage; 9, flexor pedis tendon; 10, plantar cushion;
1 1 , inflexion of wall or " bar "; 12, horny frog.
whole being incased in a protective covering which in a state
of nature is constantly being worn away and renewed. The
hoof is the homologue of the nails and claws of other mammals,
and so may be regarded as a modification of the epidermis ;
and thus viewed, its structure is at once* more readily under-
stood and more interesting. To speak from an anatomical
standpoint, the foot of the horse is made up of the terminal
LOCOMOTION.
617
i|\
Fig. 461.
Fig. 461.— Lower face of horse's foot, hoof being removed. 1, heel; 2, coronary cush-
ion; 3, branch of plantar cushion; 4, median lacuna; 5, lamina of the bars; 6,
velvety tissue of sole.
Pig. 462.— Lateral view of horse's foot after removal of hoof. 1, perioplic ring, divided
by a narrow groove from coronary cushion, 2, which is continuous with plantar
cushion, 4, and joins vascular lamina, 3, through medium of white zone.
phalanx, the navicular bone, and the lower part of the second
phalanx; certain ligaments entering into the articulations ; the
Fig. 464.
Fig. 463.— Hoof just removed from foot; side view. a. inner surface of periople, or
coronary frog-band, with some hairs passing through: «'. outer surface of same
at posterior part of foot; a", a section through the wall to show its thickness; b
toe, quarter of hoof; from b to front is outside (or inside) toe, from c to d the
outside (or inside) heel; e, frog; /, bevel, or upper niargiu of wall for reception of
coronary cushion; g, keraphylla, or horny laminae.
Fig. 464.— Hoof, with outer portion of wall removed to show its interior, a, a, peri-
ople, or coronary frog-band; b, cavity in upper part of wall for coronary cush-
ion; c, upper or 'inner surface of •'liar"; d, vertical section of wall: </'. same, at
heel; e, horizontal section of ditto; /'. horny laminae of "bar "; /". ditto of wall;
f", lateral aspect of a lamina; g, tipper or inner surface of horny sole; It. junc-
tion of horny lamina1 with the sole (the "white line"): /'. toe-stay at middle of
toe; k, upper or inner surface of horny frog: /. frog-stay: id. cavity correspond-
ing to a branch of the frog; n, ditto, corresponding to body of frog.
618
COMPARATIVE PHYSIOLOGY.
terminations of the common extensor and the perforans ten-
dons ; the lateral cartilages ; a certain amount of connective
and fatty tissue ; the hoof-secreting mechanism, together with
the hlood- vessels, nerves, lymphatics, etc., essential for all parts.
The relative size and position of parts may be gathered from
the accompanying cuts. The lateral cartilages belong to the
class known as fibro-cartilage, acting, no doubt, as perfect
buffers ; and as springs must be of no small assistance in loco-
motion.
The horny matter of the foot (hoof) owes its formation to
the cells of a tissue bearing various names in different regions,
h 7 k
Fig. 466.
Fig. 465.— Plantar or ground surface of a hoof; right foot. The interval from a to a
represents the toe; from a to 6,6, outside and inside quarters; c, o, commence-
ment of bars; d, d, inflexions of wall at heels or " buttresses "; «, lateral lacuna;
f,ff, sole; g, white line; g", ditto, between sole and bar; h, body of frog; i,
branch of frog; k, k, glomes, or heels of frog; I, median lacuna.
Fig. 466. — Horn cells from sole of hoof, a, young- cells from upper surface of sole;
b, cells from lower surface, or dead horn of sole.
but consisting of a basis of fibrous tissue abounding in blood-
vessels and nerves. The vessels from their arrangement have
determined the names given to the formative tissue, such as
villosities, villi, velvety tissue, vascular laminae, etc. It can not,
however, be too well borne in mind that these structures are
after all, only modified corium (Fig. 371).
Just as the epidermis, with its numerous layers, arises from
a modification of cells in the lower layers, resting on the vascu-
lar villi of the corium, so the hoof owes its origin to a similar
source. Thus from the velvety tissue is formed the sole and
frog ; from the perioplic ring, the periople ; and from the coro-
nary cushion, the wall (see figures).
LOCOMOTION.
619
The arrangement of the horn-tubes, the horny laminae (Figs.
467, 468), and the horn-cells is admirably adapted to form a
somewhat yielding yet very resisting structure.
Fig. 40r.— Horizontal section of junction of wall with sole of hoof. a. wall with its
horn-tubes: b. b, horny laminae projecting from wall: c.c. horn-tubes formed by
terminal villi of vascular laminae, the horn surrounding them and occupying the
spaces between the horny lamina constituting the "white line"; d, horny sole
with its tnbes.
Regarded from a mechanical point of view, for speed a
quadruped requires rather long limbs, so set on a somewhat
rigid trunk as to allow of a long as well as a rapidly repeated
stride, without undue concussion to either of the more rigid
Fig. 488. — Horizontal section of wall and horny and vascular laminre to show junction
of latter and laminellse. a, inner portion of wall with laminae arising from it: b.
vascular laminre; c, horny lamina of average length; c'. <•'. unusually short lami-
nae; c",c", laminellaa on the sides of the horny laminae; d. vascula lamina passing
between two horny ditto; d', vascular lamina passing between three horny lami-
nae; >/". lateral laminellaa; e,e, arteries of vascular lamina which have been in-
jected.
cortical parts. In the horse the fore-limbs are not attached to
the trunk by osseous connections, but the animal may be said
to be slung between its fore-limbs, all connections with the
trunk being by soft parts, as muscles, tendons, and ligaments.
620
COMPARATIVE PHYSIOLOGY.
The advantages of such an ar-
l'angenient, to an animal in which
a great deal of forward-pitching
movement occurs, in breaking
shocks are evident. The length-
ened metatarsals and phalanges
are accompanied by a very per-
fect bracing of joints by liga-
ments and tendons below, while
the shoulder is strengthened and
bound to the trunk by numerous
muscles, so that the whole, in
neatness, strength, and other
qualities required in a fleet ani-
mal, is, especially when taken in
connection with the feet, an ex-
ample of marvelous adaptation
to conditions to be constantly
met, aided in the wild species by
natural selection, and in our do-
mestic varieties by artificial se-
lection.
An examination of Fig. 470
will show the several levers
(bones) and the muscles acting on
them in one main movement of
the fore-limb.
The hind-limbs are in all gaits
of the animal its main propellers,
and these are in bony connection
with the pelvis.
Fig. 400. — Extern.il muscles of right anterior
limb (Chauveau), 1, 1, long abductor of
arm; V. its humeral insertion: 2, super-
spinatus ; 3, subspinatus; 3', its tendon
of insertion ; 4, short abductor of arm;
5, biceps; 6, anterior brachialis; 7, large
extensor of forearm; 8, short extensor
of forearm; 0, anconeus; 11, anterior ex-
tensor of metacarpus; W, its tendon; 12,
aponeurosis, separating that muscle from
anterior brachialis; 13, oblique extensor
of metacarpus; 14, anterior extensor of
phalanges; II', its principal tendon; 15, small tendinous branch it furnishes to
lateral extensor; 16, lateral extensor of phalanges; 16', its tendon; 17, fibrous
band it receives from carpus; in, external flexor of metacarpus; 19, its metacar-
pal tendon; 2D. its supracarpal tendon; 21. ulnar portion of perforans; 22, tendon
of perforans; 23, its carpal ligament; 21, its re-enforcing phalangeal sheath; 25,
tendon of the perforans,
LOCOMOTION.
621
It will not be forgotten that in joints the insheaihing carti-
lages (sometimes others more or less free), the synovial fluid,
etc., all tend to diminish friction and lessen
concussion.
We shall now describe the principal gaits
of the horse in a somewhat synoptical way.
In each gait we have to consider the
relative position of the four limbs, the
duration of each phase in the move-
ment, the length of the stride, its
rate, etc. Much that applies to
the horse holds good, of course,
of other quadrupeds.
In every gait each leg passes
from a condition of flexion to
one of extension, the degree be-
ing dependent on the speed or,
more correctly, the effort of the
animal to attain high speed or
reverse.
Pig. 470. — Internal aspect of left an-
terior limb (Chauveau). 1, pro-
longing cartilage of scapula ; 2,
inner surface of scapula; 3, sub-
scapulars; 4. adductor of fore-
arm, or portion of caput mag-
num; 7. large extensor of fore-
arm, other portion of caput mag-
num; 8, middle extensor, or ca-
put medium: 11. humeralis exter-
nns, or short flexor of forearm;
10, coraco-humeralis; 11, upper
extremity of humerus ; 12, co-
raco-radialis. or flexor brachii ;
13, lower extremity of humerus;
14. brachial fascia ; 15. anterior
extensor of metacarpus, or ex-
tensor metacarpi magnus : 16,
belly and aponeurotic termina-
tion of flexor brachii; 17. ulna:
18, ulnaris accessorius. or oblique
flexor of metacarpus: 11), inter-
na! flexor of metacarpus, or epicondylo-metacarpus; 20. ther set is wholly
radius: 21, tendon of oblique extensor: 22. large meta-
carpal-bone : 33, flexor tendons of foot : 24, suspensory relaxed. xhe
ligament; 25, internal rudimentary metacarpal bone; 26. ,-■ -i i
extensor tendon of foot; 27, metucarpo - phalangeal more UlOl'OUgim
sheath; 28, lateral cartilages of foot; 29, podophylhe. muscular move-
ments are studied the more complex, so far as the use of muscles
is concerned, are they found to be, a fact which is illustrated
when even a single muscle is weakened or paralyzed.
When the foot
rests upon the
ground before
the limb is re-
moved, it de-
scribes the arc
of a circle, or os-
cillates like a
pendulum so that
the flexors and
extensors are
used alternately
more and less ;
==^=-* though in all
movements it is
likely that nei-
622
COMPARATIVE PHYSIOLOGY.
Walking. — In tliis gait the body rests on diagonal feet alter-
nately with the two of the same side ; the center of gravity
being shifted to one side, then returned to its original position,
to be moved next to the opposite side. In drawing heavy loads
the body is supported on three limbs. The rate of movement is
one to two metres per second.
Amble. — In this mode of progression, most common in the
Fig. 471— Movements (oscillation) of an extended hind-leg (Colin). The hip-jomt
describes the arc of a circle, ABC, while the foot is on the ground, the lines A D,
B D, and C D representing the changing axis of the hind-leg.
giraffe and camel tribe, occasional in ruminants and solipeds,
the body is supported by the two legs on the same side, as in the
walk, but the two opposite legs are elevated simultaneously and
not separately. In horses this gait is often termed pacing, and
is frequently very fast. Only two strokes of the feet are heard
in this gait.
In racking the hind-leg leaves the ground sooner than the
corresponding fore-leg, hence four strokes of the feet are heard.
The Trot.— The diagonal feet act together, two strokes of the
feet being heard at each complete step. In the fast trot there
is an interval in which all four feet are in the air. The hind-
feet strike the ground in front of the fore-feet. The speed in
LOCOMOTION.
623
the fast trot may reach from eight to twelve metres per sec-
ond.
The Gallop. — The gallop may be regarded as a series of
Fig. 472. — Movements of fore-limbs of horse (Colin). While one fore-leg is describine
the movements figured above the other acts as a support. While the right fore-
foot describes the arc gh, the left shoulder describes the arc a' b' c', owing to the
impulse from extension of the hind legs. The center of gravity is advanced from
m to n, the left leg in one complete step occupying the six positions indicated at
abed ef.
jumps in which the hind-legs take the greater part, though as
in all gaits the fore-legs are not only supporters but propellers.
Fig. 473.— Various positions of the limbs in the trot (Colin).
In the perfect gallop only two strokes of the feet are heard; in
the canter or slow gallop four, in the ordinary gallop three.
According as the one or other hind-leg is extended farthest
behind the body the gallop is termed right-handed or left-
handed.
624
COMPARATIVE PHYSIOLOGY.
In the fastest gallop the length of stride may amount to six
to seven metres, and the speed to twelve to fifteen metres per
second. In such a rapid gait the contact of the one hind foot
produces a sound lengthened by the rapid impact of the fellow-
foot. The same applies to the fore-feet, hence only two sounds,
while in the other varieties of this gait the interval between the
impacts is sufficient to allow of three, or it may be four sounds.
The accompanying plate, constructed by the help of instan-
taneous photography, illustrates the different positions of a
horse in the gallop.
Sloping shoulder-blades and well-bent stifle-joints are gener-
ally recognized as of great importance to an animal intended
for high speed, and these are commonly to be met with in the
Fig. 474.— Various positions in the trot (Colin)
fleetest of horses, dogs, and other quadrupeds (Fig. 452). It
may be seen that such an arrangement permits of a length-
ened stride being taken with ease, tends to reduce concussion,
and adds to beauty of form. To this must, in part at all events,
be attributed the grace of form and fleetness of the race-horse
and the greyhound, not to mention wild animals.
A horse for heavy-draught purposes requires great muscular
power, which in turn implies a strongly developed osseous sys-
tem; and in order that this may be attained some of those
principles on which speed depends must be subordinated to
those involved in strength. As is well known, the cart-horse
and race-horse, the mastiff and the greyhound, are opposites in
build and capacity for speed. However, between these extreme
forms there are many others of an intermediate character, as
the hunter, roadster, etc. When famous race-horses are studied,
626 COMPARATIVE PHYSIOLOGY.
while the form of the animal generally agrees with what would
have been expected on mechanical principles it is a fact that
some of the fleetest horses that have ever run on the course have
not in all respects been built in conformity with them. But
it is to be remembered that a vital mechanism differs from all
others in that the whole consists of parts dependent not only as
one portion of any machine is on the other, but that every part
is energized and directed by a governing nervous system ; that
every cell is being in a sense constantly renewed, so that the
comparison between any ordinary mechanism and the body of a
living animal holds only to a limited extent. Moreover, apart
from peculiarities in the muscles of animals, to which atten-
tion has been drawn (page 205), it is well to bear in mind that not
only every animal, but every tissue has its own functional indi-
viduality ; and to this especially (as exemplified in the most im-
portant of all the tissues, the nervous) must we attribute the
undoubted fact that the speed, endurance, etc., of animals can
not be explained on mechanical principles alone — a truth to
which too little attention has hitherto been drawn. These
principles have, however, been unconsciously recognized prac-
tically, hence the great attention paid by breeders to using ani-
mals for stock purposes that have actually shown merit by their
performances.
Evolution. — It is noteworthy that with almost all quadru-
peds the gallop is the natural method for rapid propulsion. In
all animals, either bred by man to attain great speed, as the
race-horse and greyhound, or those that have become so by the
process of natural selection, the entire conformation of the
body has been modified in harmony with the changes that have
taken place in the legs and feet. This is seen in the greyhound
among domestic animals, and in the wild deer of the plain and
forest. Such instances illustrate not only the principle of
natural selection as a whole, but the subordinate one of corre-
lated growth.
Any one observing the modes of locomotion of quadrupeds,
especially horses and dogs, will perceive the advantages of the
four-legged arrangement. Not only is there a variety of modes
of progression, as walking, trotting, galloping, cantering, the
alternations of which permit of rest to certain groups of mus-
cles, with their corresponding nervous connections, etc., but on
occasion some of these animals can progress fairly well with
three legs. Sometimes it may also be noticed that a horse that
LOCOMOTION. 627
prefers one gait, as pacing, for his easy, slow movements, will
break into a trot when pushed to a higher rate of speed.
Trotting can not be considered the natural gait for high
speed in the horse, yet, by a process of "artificial selection"
(by man) from horses that have shown capacity for great speed
by this mode of progression, strains of racers have been bred,
showing that even an acquired mode of locomotion may be
hereditary ; while that galloping is the more natural mode of
locomotion of the horse is evident, among other things, by the
tendency of even the best trotting racers to break into a gallop
when unduly pushed — an instance also of an hereditary tend-
ency of more ancient origin prevailing over one more recent.
The bipedal modes of progression of birds are naturally very
like those of man.
INDEX
Abductor or sixth nerve, 582.
Abnormal urine, 421.
Accelerator nerves of heart, 258.
Accommodation of eye, 532.
Action of mammalian heart, 222.
Affections of retina, 540.
Afferent fibers, 583.
After-images, etc., 543.
Alimentary canal of vertebrate,
331.
Allantoic, 77.
Allantoic cavity, 80.
Albumins, 145.
derived, 145.
Alterations in size of pupil, 533.
Amble, 624.
Amnion, 76.
Amoeba, 13.
Amylolytic action of saliva, 297.
Animal body, 28.
Animal foods (table), 277.
Animal heat, 445.
Animals deprived of cerebrum, 482.
Anaemia, 117.
Anomalies of refraction, 536.
Apncca, 395.
Apparatus used for stimulation of
muscle, 179.
for transmission of muscular move-
ments by tambours, 182.
Asphyxia, respiration and circulation
in, 399.
Auditory ossicles, 559.
Auditory impulses, 565.
sensations, etc., 567.
Automatism, nervous system, 211.
Automatic functions of spinal cord,
475.
Bacteria, 18.
Barking, 402.
Bawling, neighing, braying. 403.
Beat of the heart and its modifica-
tions, 248.
Bell-animalcule, 21.
Bile, digestive action of, 303 .
salts, 302.
pigments, 302.
Biology, general, 1.
table, 4.
Blastodermic vesicle, 78.
Blood, 154.
cells, 158.
cells, decline, and death, 1 60.
chemical composition of, 160.
pressure, 223-227.
flow, 227.
Bone, 604.
Botany, 4.
Brain. 481.
Capillaries. 264.
Carbon-dioxide of blood, 389.
Carbohydrates, 146.
Cardiac movements, 231.
sounds, 234.
630
COMPARATIVE PHYSIOLOGY.
Cartilage, 609.
Causes of the sounds of the heart, 235.
Cell, 5.
the male, 61.
Cellulose, 9.
Cerebellum, 508.
Cerebral cortex, 497.
Cerebro-spinal system of nerves, 580.
and sympathetic systems, relations
of, 588.
Certain tissues, 603.
Characteristics of proteids, general,
144.
of blood-flow, 226.
of secretion of different glands,
298.
Changes in muscle during contrac-
tion, 189.
produced in food in alimentary
canal, 354.
in circulation after birth, 129.
Chemical constitution of animal body,
142.
changes in muscle, 189.
composition of blood, 160.
Chemistry of unicellular plants, 9.
Chondrin, 145.
Chorion, 79.
Chronographs, 176.
Ciliary movements, 179.
(ophthalmic lenticular), ganglion,
584.
Circulation of blood, 214.
in mammal, 219,
under microscope, 224.
in brain, 500.
Circumstances influencing character
of muscular and nervous activ-
ity, 199.
Classification of animal kingdom, 34.
of proteids, 145.
Clinical and pathological re blood,
167.
Coagulation of blood, 163.
Coitus, 129.
Color-vision, 543.
Comparative re blood, 154, 172.
unstriped muscle, 202.
blood-pr,essure, 224.
cardiac pulsation, 240.
circulation, 244, 267.
digestion, 280, 310, 357.
metabolism, 436.
diet, 438.
digestive juices, 298.
digestive organs, 324, 337.
bile, 303.
feeding experiments, 441.
fats and carbohydrates, 442.
animal heat, 445.
spinal cord, 477.
cerebral convolutions, 485.
muscular sense, 524.
vision, 536-551.
hearing, 568.
senses of smell and taste, 674, 578.
voice, 598.
locomotion, 614.
swallowing, 336.
vomiting, 340.
movements of lymph, 344.
respiration, 376, 398.
haemoglobin, 389.
respiratory movements, 402.
respiration by skin, 412.
perspiration, 413.
expulsion of urine, 426.
Comparison of inspired and expired
air, 382.
Composition of serum, 131.
of corpuscles, 162.
of milk, 275.
Conclusions re unicellular plants, 10,
protococcus, 12.
unicellular animals, 15.
nervous system, 212.
heart, 257.
salivary secretion, 314.
INDEX,
631
Condiments, 278.
Connective tissue, 603.
Contractile tissues, 1*71.
Connection of one part of brain with
another, 491.
Construction of fat, 432.
Conditions under which gases exist
in blood, 384.
Co-ordination of two eyes in vision,
544.
Coughing, 401.
Corpuscles, 156.
action of the, 22Y.
Corpora quadrigemina, 506.
Corpus striatum and optic thalamus,
504.
Cranial nerves, 580.
Crying, 402.
Decussation, 4*71.
Defecation, 338.
Deglutition, 333.
Dentition of domestic animals (table),
290-296.
Development of embryo, 95.
of vascular system in vertebrates,
108.
of urogenital system, 112.
Dextrin, 146.
Diet, 437-439.
effects of gelatin in, 441.
effects of salts, water, etc., 443.
Digestion of food, 274.
Digestive juices, 297.
action of bile, 303.
organs, movements of, 332.
Dioptrics of vision, 531.
Discoidal placenta, 83.
Domesticated animals, 47.
Dyspnoea, 396.
Effects of gelatin in diet, 411.
Efferent nerve-fibers, 583.
Entrance and exit of air, 370.
Elasticity of muscle, 189.
Elastin, 145.
Elastic tissues, 604.
Electrical phenomena of muscle, 191.
organs, 197.
Embryological re digestion, 279.
brain, 510.
vision, 528.
Embryo, development of, 95.
Embryology, applied to evolution, 45.
Embryonic membrane of birds, 74.
Endocardiac pressure, 236.
Energy of animal body, 443.
Epiblast, 98.
Epithelium, 7.
Evolution, 42.
re reproduction, 93.
circulation, 268.
digestion, 363.
respiration, 404.
metabolism, 450.
spinal cord, 478.
brain, 511.
vision, 552.
hearing, 571.
voice, 600.
locomotion, 627.
Estimation of size, etc., of objects,
548.
Excretory function of skin, 411.
Excretion of perspiration, 412.
by the kidney, 415.
Experimental facts, 185.
re nervous system, 210.
digestion, 305.
spinal nerves, 580.
Eustachian tube, 562.
Eye, accommodation of, 532.
optical imperfections of, 536.
protective mechanisms of, 549.
Facial nerve, 581.
and laryngeal respiration, 374.
Fat, construction of, 432.
632
COMPARATIVE PHYSIOLOGY.
Fats, 145. .
and carbohydrates, 442.
Fatigue, 199.
Feeding experiments, 439.
Features of an arterial pulse tracing,
242.
Fertilization of ovum, 63.
Fibrin, 145, 164.
Faeces, 352.
Foetal circulation, 125.
later stages of, 109.
membranes of mammals, 78.
Food, digestion of, 274.
stuffs, 274.
Foods, animal, table of, 277.
vegetable, table of, 277.
Foreign gases in respiration, 391.
Forced movements, 484.
Fossil and existing species, 46.
Fresh-water polyps, 23.
Fungi, 15.
Functional variations, 204.
Functions of cerebral convolutions,
485.
of other portions of brain, 504.
Gastrula, 68.
Gastric juice, 299.
Gelatin, 145.
Geographical distribution, 46.
Globulins, 145.
Glosso-pharyngeal or ninth nerve,
585.
Glycogen, 428.
uses of, 429.
Glycocholic acid, 302.
Gout, 139.
Graafian follicle, 58.
Graphic method and study of muscle
physiology, 176.
Gustatory fibers, 582.
Haemoglobin and its derivatives, 385.
Hearing, 557.
Heart, 231.
of various animals, 245, 246.
beat in cold-blooded animals, 250.
causation of beat of, 253.
influence of vagus nerve on, 253.
accelerator nerves of, 258.
in relation to blood-pressure, 260.
Hen's egg, 69.
Hiccough, 402.
History of blood-cells, 158.
Hydra, 26.
Hypoblast, 98.
Hypertrophy, 265.
Hypnotism, 502.
Hypoglossal or twelfth nerve, 588.
Impulse of heart, 232.
Influence of blood-supply, 199.
of temperature, 201.
of vagus nerve upon heart, 253.
of condition of blood in respira-
tion, 393.
of respiration on circulation, 396.
of nervous system on metabolism,
452.
Inhibition of reflexes, 469.
Inorganic food-stuffs, 144, 274.
Inosit, 145.
Inorganic salts, 420.
Instincts, 42.
Investigation of heart - beat from
within, 233.
Intestinal movements, 337.
Irritability of muscle and nerve, 175.
Juices, digestive, 297.
Keratin, 145.
Lactose, 146.
Law of periodicity or rhythm in na-
ture, 37.
of habit, 41,405.
of rhythm, 269.
INDEX.
633
Laws of retinal stimulation, 541.
Laughing, 401.
Living things, 2.
Living and lifeless matter, 32.
Lymphatic system, 342.
Lymph and chyle, 343.
Locomotion, 610.
Maltose, 146.
Mammalian heart, 215, 222.
Man's place in animal kingdom, 36.
Medulla oblongata, 509.
Membrana tympani, 558.
Mesoblast, 98.
Metadiseoidal placenta, 84.
Metabolism, 27, 428.
of liver, 428.
of spleen, 429.
influence of nervous system on,
452.
summary of, 45S.
Metazoa, 5, 53.
Milk, composition of, 275.
sugar, 276.
Mimicry, 45.
Molds, 15.
Morphology of unicellular plants, 9.
of protococcus, 12.
of unicellular animals, 13.
applied to evolution, 45.
Motor oculi nerve, 581.
Movements of digestive organs, 332.
stomach, 336.
lymph, 344.
Mucin, 145.
Mucor mucedo, 17.
Mullerian duct, 115.
Multicellular organisms, 23.
Muscular contraction, 185.
Muscle tone, 189.
Muscular work, 198.
Muscles of respiration, 373.
of middle ear, 561.
Muscular sense, 524.
Nasal or spheno-palatine ganglion,
584.
Nature of act of secretion, 318.
Nerve-cells, 209.
supply (voice), 596.
Nervous mechanism, 134.
system, 208.
inhibition, 212.
system in relation to heart, 249.
supply — digestion, 333.
system in relation to respiration,
393.
Nitrogen of blood, 389.
Nitrogenous metabolites, 147.
crystalline bodies, 420.
equilibrium, 440.
Non-nitrogenous metabolites, 147.
organic bodies, 420.
Non-crystalline bodies, 145.
Notochord, 99.
Nucleus, 5. i '
Nucleolus, 5.
Nuclear division, 6.
Nuclein, 145.
Nutrition of ovum, 123.
Ocular movements, 544.
(Estrum, 121.
Optical imperfections of the eye, 536.
Origin of forms of life, 42.
of spermatozoon, 61.
of fowl's egg, 70.
Organic evolution reconsidered, 137.
food-stuffs, 144-274.
Otic ganglion, 585.
Ovulation, 120.
Ovum, 55.
origin and development of, 57.
changes in, 58.
Oxy-ha3moglobin, 386.
Paleontology, 46.
Pancreatic juice, 305.
Parasitic organisms, 15.
634
COMPARATIVE PHYSIOLOGY.
Parturition, 128.
Pathological re food-stuffs, 147.
muscle, 197.
circulation, 244, 265.
bile, 318.
stomach, 330, 337.
lymphatics, 352.
faeces, 354.
digestion, 359.
respiration, 379, 401, 403.
skin, 411.
urine, 423.
expulsion of urine, 426.
metabolism, 435, 443.
temperature, 449.
spinal cord, 471.
brain, 508, 509.
muscular sense, 524.
vision, 536, 554.
hearing, 563.
spinal nerves, 580.
third and other nerves, 582, 585,
587, 588.
voice, 598.
Peculiar respiratory movements, 401.
Peptones, 145.
Pendulum myograph, 184.
Perspiration, excretion of, 412.
Periods of gestation, 127.
Pfliigcr's monograph, 181.
Physiology of unicellular plants, 10.
of protococcus, 12.
unicellular auimals, 13.
nerves, 195.
Physiological aspects of development,
118.
research and reasoning, 148.
Placenta, 82.
discoidal, 83.
metadiscoidal, 84.
zonary, 89.
diffuse, 89.
polycotyledonary, 89.
Simple, 90.
Placenta multiple, 90.
microscopic structure of, 90.
Pneumogastric nerve, 586.
Polyps, 23.
Pressure, endocardiac, 236.
Pressure sensations, 522.
Proteids, general characteristics of,
144.
of milk, 276.
Proteus animalcule, 13.
Protococcus, 11.
Protozoa, 5, 53.
Protective and excretory function of
skin, 40S.
Protective mechanism of the eye, 549.
Protoplasm, 3.
Proximate principles, 144.
Pulse, 241.
the venous, 244.
Psychological aspects of vision, 543.
Quantity and distribution of blood,
163.
of blood, influence on blood-press-
ure, 261.
of air respired, 378.
Reflex functions of the spinal cord,
466.
Regulation of temperature, 447.
Relations of cerebro-spinal and sym-
pathetic symptoms, 588.
Relative value of food-stuffs, 444.
time occupied by cardiac cycle, 237.
Removal of digested products from
the alimentary canal, 341.
Reproduction, 51.
Retinal stimulation, laws of, 541.
Respiratory system, 366.
rhythm, 379.
sounds, 81.
Respiration, muscles of, 373.
facial and laryngeal, 374.
types of, 375.
in blood, 383.
INDEX.
635
Respiration in tissues, 391.
Respiration and circulation in as-
phyxia, 399.
in mammal, 405.
by skin, 412.
Rigor mortis, 206.
Rhythm, 37.
law of, 269.
Rudimentary organs, 46.
Salts, 144, 276.
inorganic, 420.
Saliva, 297.
amylolytic action of, 297.
Scar-tissue, 139.
Serum, composition of, 161.
Secretion as a physiological process,
of salivary glands, 311.
by stomach, 315, 321.
of bile and pancreatic juice, 316.
nature of the act, 318.
of mine, 421.
Secretory fibers, 582.
Segmentationand subsequent changes,
64.
Self-digestion of digestive organs,
323.
Sense organs, 31.
Sexual selection, 43.
Separation of muscle from central
nervous system, 201.
Semicircular canals, function of, 484.
Sighing, 402.
Senses, general remarks, 516.
of smell and taste, 573.
Skin as an organ of sense, 520.
Smell, 573.
Sleep, 501.
Sobbing, 402.
Solidity, 548.
Soap, 146.
Sneezing, 402.
Special considerations re muscle, 208.
circulation, 265.
Special considerations re digestion,
359.
metabolism, 450.
spinal cord, 477.
brain, 510.
vision, 551.
hearing, 568.
voice, 600.
Specific gravity of urine, 419.
Spermatozoa, 61.
Spbygmograph, 243.
Spinal cord, general, 461.
reflex functions of, 466.
as a conductor of impulses, 469.
automatic functions of, 475.
nerves, 579.
accessory, 587.
Sporangia, 17.
Starches, 147.
Study of metabolic processes, 436.
Submaxillary ganglion, 585.
Succus entericus, 307.
Sugars, 146.
Summary of biology, 9.
of evolution, 50.
of reproduction, 93.
development of the embryo, 135.
physiological research, etc., 152.
blood, 169.
muscle and nerve, 205.
circulation, 269.
blood-cells, 158.
digestion, 364.
respiration, 405.
perspiration, 413.
urine, 426.
metabolism, 458.
voice, 001.
Synoptical re spinal cord, 479.
brain, 513.
skin, 525.
vision, 564.
hearing, 572.
Swallowing, 335.
636
COMPARATIVE PHYSIOLOGY.
Tactile sensibility, 523.
Tambour of Marey, 183.
Taste, 575.
Teeth, 2S4.
structure and arrangement of the,
286.
Temperature, regulation of, 447.
Tetanizing key of Du Bois-Reymond,
181.
Tetanic contraction, 187.
Thermal changes in contracting mus-
cle, 195.
sensations, 522.
Tissues, 8.
Trigeminus, trifacial or fifth nerve,
583.
Trochlear or fourth nerve, 582.
Trot, 624.
Types of respiration, 375.
Unicellular plants, 9.
animals, 13.
with differentiation of structure, 21.
Unstriped muscle, 202.
Urea, 147.
Urine considered physically and chem-
ically, 419.
abnormal, 421.
Urine, secretion of, 421.
expulsion of, 424.
Variations in cardiac pulsation, 239.
of average tempei'ature, 446.
Vasomotor nerves, 262.
Vegetable foods (table), 277.
Velocity of blood and blood-press-
ure, 223.
Venous pulse, 244.
Vision, 526.
dioptrics of, 531.
psychological aspects, 543.
Visual sensations, 538.
angle, 542.
Voice, 593.
Vomiting, 339.
Vorticella, 21.
Walking, 623.
Water, 443.
Wolffian duct, LI 5.
Work of the heart, 238.
Yeast, 9.
cells, 10.
Yawning, 402.
Zoology, 4.
THE END.
October, IS93.
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